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This section contains definitions of terms that are commonly used in gameplay discussions.
- Anti-Air - an attack that can reliably hit opponents out of the air.
- Chip Damage - damage incurred to a player's life meter when blocking an attack (often special, EX or Level 3); guard FX is orange in this case.
- Combo - a series or sequence of attacks such that when the first hit connects, the subsequent attacks cannot be blocked or avoided.
- EX moves - powered up special attacks, that do considerably more hits and damage (if any) and may as well sport additional properties; require 1 super stock to perform.
- Guard Crush - a state in which the player is temporarily unable to act after their guard meter has been depleted because of guarding too many attacks.
- Guard Damage - same as Chip Damage.
- Juggle - a special type of combo that allows the player to combo the opponent while they are airborne.
- Level 3 - nickname for a super move, unique to each character, that requires 3 super stocks and a unified A+B+C command (in most cases) to perform.
- Normals - same as Normal Attacks, the least damaging attacks in player's posession, usually deal no chip damage, so the guard FX is blue.
- OTG (Off The Ground) - ability to hit a downed opponent, thus extending a combo.
- Overhead - an attack that must be blocked while the character is standing.
- Tick Damage - same as Chip or Guard Damage.
- Tick Throw - a maneuver used to throw an opponent immediately after their hit stun or block stun has ended.
- Throw Break - a common technique, that allows a player to escape opponent's ground throw after it's been initiated; is performed with the same input as opponent's throw.
Official Story Introduction
In the year 20XX AD in one of Tibet's most secluded regions, the Tsangpo Canyon, there was a barely reported cave-in incident. Several months later an organization calling itself "New Order of Knights Templar", by means of simultaneous terrorist outbreaks, turned the world into chaos. And now each faction, taking advantage of this confusion, begin a fight in the shadows for the New World's supremacy! (*)
- (*) The original word used here 暗闘 (antou) means "secret feud" but is also a synonym of Danmari. 
- "In Kabuki, the word danmari means also "fight in the dark". It is a pantomime, with actors moving slowly in the dark. There are 2 kinds of Danmari: 1) Sewa danmari: a danmari scene included in a sewamono drama. It is a highly choreographed fight in the dark, with a precious object moving from hand to hand up to the final pose. 2) Jidai danmari: ceremonial item performed as an independant play. In the past, a jidai danmari was a way to present a full troupe to the audience. It is a large-scaled pantomine made up of a succession of kata performed simultaneously by several actors, with a music accompaniment but without any dialogue." Taken from 
The terms here describe only the game's background story, and are in no way related to gameplay. Corrections are welcome.
- Agartha - アガルタ (Tibet) | Neuland ノイラント (Antarctica)
- Ancient cities where human-like entities known as Gods once lived. Possibly the origin of the Blitz Motor.
- Agartha was said to be a subterranean kingdom that was located under the Himalayas in Tibet based on the theory of a hollowed Earth, and was supposed to hold immense wisdom and wealth. 
- Blitz Motor - 電光機関 (Denkou Kikan)
- The mcguffin of the ABK series. Some kind of engine that can be installed into equipment and people to give them super human abilities. Powered by ATP , causing it to shorten the lifespan of those with one built into them, with the exceptions of the perfect Blitz Motors. Supposedly created by the Ancient Gods that lived in Agartha/Neuland.
- Ancient Gods / Ancients - 古代人 (Kodaijin)
- Term used to refer to the ancient being(s) that lived in Agartha. Possibly the original creators of the Blitz Motor, and considered to be a more evolved version of humans by the Perfecti Cult. EN-Eins is apparently cloned off of a God. It's likely that Valkyria is one of these beings.
- Perfecti Cult / Cult of Perfection - ペルフェクティ教団 / 完全教団 (Perfecti Kyoudan / Kanzen Kyoudan)
- A secret religious society founded by Mycale in Southern France during the 12th century. Its main goal is the salvation of the souls of all of humankind by destroying their physical bodies, a plan called the Pneuma Project - プネウマ計画 (Pneuma Keikaku). To this end Mycale and the Cult have been shaping the most important events of the world's history since the Middle Ages. Gesellschaft's base in Tibet, the discovery and producion of the Blitz Motor, the creation of the clone soldiers and the worldwide attack of the New Order of Knights Templar are some the means towards that end.
- The Perfecti Cult is heavily based on Catharism in many of its principles and beliefs. Originated in Southern France (the Languedoc region) in the middle of the 11th century, Cathars believed in two gods: one the good god, the creator of the spiritual realm and the other an evil god (refered as Satan or Rex Mundi) creator of the physical world. 
- The spirit was of utmost importance to the Cathars and the body and the world were considered tainted by sin. In order to ascend to the spiritual realm one had to renounce the material self completely, until one was prepared to do so, their souls would be stuck in a cycle of eternal reincarnation, condemned to live on the corrupt Earth.
- These beliefs were considered heretical and the antithesis of the Catholic Church which led to the Albigensian Crusade (1209–1229) initiated by Pope Innocent III to eliminate Catharism in Languedoc. This was followed by the instauration of the Inquisition that completely eradicated the Cathars in 1350. 
- Perfecti - ペルフェクティ / 完全者 (Kanzensha)
- Term used to refer to those in the Perfecti Cult who mastered the principle of reincarnation and can resurrect in another person's body. Mycale as the leader of the cult is one these beings. It's stated in the story that even though Murakumo and Adler can resurrect they haven't reached this status yet. They are still tied to their bodies since they can only resurrect by transmigrating to their respective clones unlike Mycale who can do it freely.
- This title is based on the similar one used by the Cathar religion in the Middle Ages. Those who followed a lifestyle of extreme austerity and renunciation of the world were considered "Perfect". The Perfecti were the Cathar spiritual elite, highly respected by many of the local people, leading a life of austerity and charity. 
- Gesellschaft / Ahnenerbe - ゲゼルシャフト / アーネンエルベ
- Mycale used the Third Reich's Ahnenerbe (named Gesellschaft after the war apparently) to conduct research and development to further the Perfecti Cult goals. Ranging from building a base of operations in Tibet, the excavation in the Tsangpo Canyon to uncover Agartha and the Blitz Motor, the clone soldier project that gave Gesellschaft its army of Elektrosoldats, the creation of the living weapon Blitztank, and the infiltration of the Asian governments and organizations by Murakumo clones among other things.
- Led by Murakumo, they were the main armed forces of the cult until the destruction of their base in Tibet at the hands of Akatsuki according to EE story.
- New Order of Knights Templar - 新聖堂騎士団 (Shin Seidou Kishidan)
- Led by Valkyria, the order is the new armed forces of the Perfecti Cult replacing Gesellschaft after the events of ABK/AA with its main base located in Antarctica in a city similar to Agartha named Neuland. It still retains many of the weapons developed by Gesellschaft including the army of Elektrosoldats and Blitztank as its main forces along with the new flying troops called Tempelritter (Temple Knights) cloned from Valkyria.
- It's unclear if this organization has any relation to the original Knights Templar that were active from 1119 until its dissolution in 1312 under charges of heresy. 
- Clones - クローン / 複製體 (Fukuseitai)
- Akatsuki, Fritz, Murakumo and Dà Wèi use 複製體 (fukuseitai) throughout the series to refer to the clones produced by Gesellschaft (e.g. Murakumo clones, and Elektrosoldats). This term doesn't have a direct translation: (1) 複製 (fukusei) reproduction; duplication; reprinting (2) 體 the body; substance; object; reality. The common term for clone in Japanese is クローン which is what Adler, Elektrosoldat, Perfecti and Valkyria use. It seems to imply that the Japanese in this setting didn't get to use a loanword for the term for some reason. The same is true for Chinese which uses an equivalent loanword for the term (克隆 kèlóng).
- The Big Boss - 大当家 (Dàdāngjiā)
- The leader of the the Black Hand mafia. This title is spelled in Chinese from 大 (dà) big; huge; large; major; great; older; eldest and 当家 (dāngjiā) 1) to be the one in charge of the family; 2) to call the shots; to be in charge. This position was first held by Yin Xu in ABK/AA and now by Marilyn Sue in EE.
- Holy Club - 神聖クラブ (Shinsei Club)
- This organization seems to be a mixed bag of many different religion references: the palace could refer to St. Peter's Basilica in the Vatican City ; the name is very similar to the Holy Club which was the foundation of the Methodist Church ; they use specifically 異端審問  (Itan Shinmon - The Inquisition ) to refer to Anonym's position within the Holy Club; Anonym's orders in ABK/AA are stated to come directly from the Pope; and considering reincarnation a sacrament which are a fixed number of rites in most religions that have them, the only exception being the Eastern Orthodox Church with the notion of Holy Mysteries.
This is a direct translation from the official ABK site taken from the section Words which details the meaning of various terms used throughout the games. The bullet points below each give further explanation if needed.
- Year 266X AD - 紀元266X年
- The year is 266X AD in the Japanese national calendar . It corresponds to 200X AD in the Gregorian calendar .
- The Japanese Imperial year system (皇紀 kōki) uses the year of Emperor Jinmu's  ascension to the throne as its zero point which happened in 660 BC.
- M60 Blitz Uniform [Roku Zero Shiki Denkou Hifuku] - 六○式電光被服
- The 2660 AD (Imperial) model, which means the one adopted in 2000 AD (Gregorian).
- Literally "lightning clothes". The 六○式 comes from military weapon terminology, meaning M60 (also Model 60) which is the latest version of the Blitz Uniform (as used by Murakumo). This is also translated as "Blitzanzug" in Akatsuki's profile in Under Night In-Birth.
- German Military Affairs Delegation Dispatch - 遣独軍事使節団
- During the Great War, submarines were used to carry out Japanese-German technology exchange. Before the outbreak of the war the "Indian Ocean - Cape of Good Hope" route was used, but during the closing years of the Great War navigation was near impossible.
- Hibernation Control Unit [Toumin Seigyo] - 冬眠制御
- The technology used to control metabolism and lower body temperature. It also improves the immune system.
- Akatsuki - アカツキ
- Codename originated from "Akatsuki Unit" which was the alias of an Imperial Army naval force.
- This is the description from the Japanese Wikipedia article related to 暁部隊 (Akatsuki Unit): 
- "The Shipping Command Unit (Senpaku Shireibu Tai) was an organization of the Imperial Japanese Army that supervised the transportation of troops, goods, etc. during the war. It was assigned the Unit Character Name: "Akatsuki Unit". More popularly know as "Akatsuki"."
- The Imperial Japanese Army gave codenames, based on a guideline called Tsūshōgō , that assigned a character (kanji) and a number for each unit. In this case 暁 meaning dawn, daybreak.
- Gesellschaft - ゲゼルシャフト
- Its real meaning being closer to "Society". It originates from "German asociation for the spiritual inheritance of protohistoric ancestors" to be commonly referred to as "Ahnenerbe".
- アーネンエルベ "Ancestors Inheritance" from Ahnen (ger: ancestors, forefathers) and Erbe (ger: inheritance, heritage, legacy). The phrase is a strong reference to the motivation behind the Perfecti Cult, Gesellschaft and the New Knights Templar throughout the series: "protohistoric ancestors" the ancient Gods (like Valkyria) and "spiritual inheritance" Mycale's endgoal of global perfection with the Pneuma Project.
- Hēiháizi - 黒孩子（ヘイハイズ）
- An unregistered illegitimate child not declared for being born outside the one-child policy.
- Literally meaning "Black Child". 
- Professional Assassin - 職業兇手
- A hitman.
- Could be also read as "Zhíyè Xiōngshǒu" in Chinese.
- Hēishèhuì - 黒社会
- Underworld organization, mafia.
- From chinese 黑社会 (hēishèhuì) criminal underworld; organized crime syndicate.
- Black Hands - 黒手会（ブラックハンズ）
- Taking advantage of Hong Kong handover to mainland China, the local mafia expanded into the continent. The origin of the name comes from its assassins black gloves. It's unrelated to the Serbian Black Hand Society. 
- The name is very similar to the Chinese word for mafia: 黑手党 (hēishǒudǎng).
- State Council - 国務院
- The Chinese government.
- From 国务院 (Guówùyuàn) State Council (of the People's Republic of China). 
- 8th License Holder - ８番目の許可証保持者
- License given to the strongest agents authorizing homicide.