C9

C9, C09 or C-9 may refer to:

Biology, medicine, and chemistry

 * C9 (Complement component 9), a protein
 * ATC code C09, a subgroup of the Anatomical Therapeutic Chemical Classification System
 * C09, ICD-10 code for malignant neoplasm of tonsil
 * Carbon-9 (C-9 or 9C), an isotope of carbon

Military and weapons

 * Hi-Point Models C9 and C9 Comp handguns
 * C9 LMG, Canadian light machine gun
 * C9, an ID for the German Nachtjagdgeschwader 5 air squadron in World War II

Music

 * C9, a note five octaves above Middle C
 * C9, a C ninth chord

Organizations

 * Cloud9, an American esports organization
 * C9 League, an association of Chinese universities
 * The Council of Cardinal Advisers, an advisory body to the pope, originally comprising nine members
 * C9 Entertainment, a South Korean entertainment company and record label

Transportation

 * Cierva C.9, a 1927 British experimental autogyro
 * HMS C9, a British submarine
 * Ford C-9, a US military designation for the Ford Trimotor aircraft
 * McDonnell Douglas C-9, a US Air Force transport aircraft based on the civilian DC-9
 * USS Montgomery (C-9), a US Navy cruiser
 * C9, the IATA code for Cirrus Airlines
 * Sauber C9, a Le Mans racing car
 * C9 engine, by Caterpillar Inc.
 * C-9 (Cercanías Madrid), a commuter rail line in Madrid
 * LNER Class C9, a class of 2 British steam locomotives rebuilt from C7s in 1931
 * SL C9, a type of rolling stock used in the Stockholm metro

Other uses

 * C9, an ISO 216 standard paper size
 * C9, a holiday light bulb size
 * C9, a sportswear line by Champion