# C2

C2 or a derivative (C-2, C$^2$, etc.) may refer to:

## Mathematics and physics

* C$_2$, one of the common notations for the cyclic group of order 2
* C$^2$ differentiability class
* C$^2$ or $\mathbb {C} ^{2}$, the complex coordinate plane
* c$^2$ the square of the speed of light (in the mass–energy equivalence formula)

## Biology

* C2 domain, a protein structural domain
* C2 regulatory sequence for the insulin gene
* Apolipoprotein C2, a human apolipoprotein
* In human anatomy, C2 may refer to:
** Cervical vertebra 2, the axis, one of the cervical vertebrae of the vertebral column
** Cervical spinal nerve 2
* Chlorophyll c2, a form of chlorophyll
* Complement component 2
* Procyanidin C2, a plant phenolic compound
* Prodelphinidin C2, a plant phenolic compound
* Vitamin C2, a name sometimes used for Choline
* the ATC code for Antihypertensives, a subgroup of the Anatomical Therapeutic Chemical Classification System
* C$_2$ fragments, one of the types of products of catabolism pathways
* Haplogroup C-M217, also known as C2, a Human Y-chromosome DNA haplogroup

## Chemistry

* C$_2$, Diatomic carbon, a molecule made of two carbon atoms
* C2=, ethylene, a two carbon alkene

## Codes or abbreviations

C2 may be a code or abbreviation for:
* C$^2$ Aquarii, a star
* C$^2$ Centauri, a star
* C2 (Code Geass), an anime character
* C2 explosive, a form of plastic explosive
* C2 pylon, a type of high voltage pylon
* C2 radio class, a class in FM broadcasting in America
* C2 (classification), a para-cycling classification
* Group C2, the secondary class of Group C sports car racing
* BRDC C2 Championship, a short lived sports car racing series which ran from 1988 to 1990
* Nimrod NRA/C2, a 1982 Group C racing car
* C2, an international standard paper size defined in ISO 216 (458×648 mm)
* C2, a level in the Common European Framework of Reference for Languages
* C2, the NRS social grade between C1 and D
* Command and control, the exercise of authority by a commanding officer over military forces in the accomplishment of a mission
* Air Luxor IATA airline code
* C2 Pictures, a film company related to Carolco Pictures
* Bill C-2, various legislation of the Parliament of Canada
* C2, or C$_2$ a note-octave in music, aka Low C

## Computing

* C2, a computer security class, defined in the Trusted Computer System Evaluation Criteria
* C2, a bytecode compiler that is part of the HotSpot virtual machine implementation
* C2 (protocol), a 1985 Commodore file transfer protocol
* C2 error, a read error of a compact disc
* C2.com or WikiWikiWeb, the first user-editable website
* Cryptomeria cipher, a cipher used in digital rights management
* Command and control (malware)
* Codec 2, speech compression codec.

## Transportation and space

* C2 class Melbourne tram
* Bavarian C II (Ostbahn), an 1862 German steam locomotive
* Bavarian C II.1, an 1857 German steam locomotive
* Bavarian C II.2, an 1861 German steam locomotive
* Chevrolet Corvette (C2), the generation of the Chevrolet Corvette produced between 1962 and 1967
* Citroën C2, a car produced by Citroën in 2004
* Cluster II (spacecraft), an ESA space mission
* Ford C2 platform, an automobile platform produced by Ford since 2018
* GNR Class C2, a British 4-4-2 steam locomotive class
* LNER Class C2, classified C1 during GNR ownership
* Kawasaki C2, a bike
* London Buses route C2
* LB&SCR C2 class, an 1893 British 0-6-0 steam locomotive
* Circumferential Road 2 or C-2, an arterial road of Manila, Philippines
* Saf-T-Liner C2, a school bus built by Thomas Built Buses
* Saturn C-2, a 1960 conceptual American launch vehicle
* Type C2 ship, a type of all-purpose cargo ship
* C2 (yacht), a luxury motor yacht
* Central Circular Route (Shuto Expressway) in Tokyo, Japan
* Mei-Nikan Expressway, route C2 in Nagoya, Japan

## Military and aviation

* Command and control, the exercise of authority and direction by a commanding officer
* C-2 (poison), developed in the Poison laboratory of the Soviet secret services
* C2A1, Squad Automatic Weapon variant of the FN FAL formerly in Canadian Service
* ACAZ C.2, a Belgian prototype fighter aircraft of the 1920s
* AEG C.II, a German World War II armed reconnaissance aircraft
* Aeronca C-2, an American airplane
* AGO C.II, a German reconnaissance biplane of World War I
* Cierva C.2, a 1921 Spanish experimental autogyro
* DFW C.II, a World War I German reconnaissance aircraft
* Fokker C-2, the American military version of the Fokker F.VII aircraft
* C-2 Greyhound, the Grumman cargo aircraft of the United States Navy
* HMS C2, a British C class submarine of the Royal Navy
* Kawasaki C-2, a military transport aircraft
* LFG Roland C.II, a 1915 advanced German reconnaissance aircraft
* LVG C.II, a 1910s German two-seat reconnaissance biplane
* USS C-2 (SS-13), a C-class submarine of the United States Navy
* USS Charleston (C-2), a protected cruiser of the United States Navy

## Other

* C2, UEFA Cup Winners' Cup in football, later merged with UEFA Cup (now UEFA Europa League)
* C2 Pictures, a film studio
* C-2 visa, a nonimmigrant visa which allows individuals to travel to and from the headquarters of the United Nations
* Coca-Cola C2, a soft drink launched by The Coca-Cola Company in 2004
* Conduit 2, a 2011 first-person shooter video game
* AEW Continental Classic, a professional wrestling tournament