C21

C-21, C 21, or C21 may refer to:
 * C21 Sako TRG M10 Sniper Weapon System, Canadian Armed Forces sniper rifle.
 * C21 (group), a Danish boy band
 * C21 (album), 2003
 * C-21 Dolphin, a flying boat used by the United States Army Air Corps
 * HMS C21, a 1909 British C class submarine
 * Learjet C-21, an American military transport aircraft
 * Sauber C21, a 2002 racing car
 * Space Adventures C-21, a Russian design for a suborbital rocket plane for Space Adventures
 * Anal cancer (ICD-10 code: C21)
 * Center Game (Encyclopaedia of Chess Openings code: C21)
 * Caldwell 21 or NGC 4449, an irregular galaxy in the constellation Canes Venatici
 * Carbon-21 (C-21 or 21C), an isotope of carbon
 * Catch 21, a television game show based on blackjack
 * HMS Glasgow (C21)