C9

C9, C09 or C-9 may refer to:

Medicine/biology

• 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

Military/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 (esports organization)
• C9 League, a Chinese association of universities

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.

Other uses

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