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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
266X AD (Kōki)...
A submarine that sank in the Arctic Ocean, while transporting a new weapon during the closing years of the Great War, has suddenly reemerged.
The one who appeared from inside it, was Akatsuki, a former Imperial Army technical officer dispatched for technical research.
And now in the world half a century later, a fight to the death over the highly classified weapon Blitz Motor begins......!
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.