C-21, C 21, or C21 may refer to: * C21 (band), 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)