• C21, a boy band from Denmark
• 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, a United States military version of the Learjet 35A
• Sauber C21, a 2002 racing car
• Space Adventures C-21, a Myasishchev Design Bureau created design for a suborbital rocketplane for Space Adventures

and also :

• Century 21
• Anal cancer ICD-10 code