C26 stands for Colonic Adenocarcinoma 26
C25K stands for Couch to 5k (running)
C25 stands for Control for 25 Days (research test group)
C24 stands for Concentration at 24 Hours (also seen as C24 h)
C21 stands for Century 21 Real Estate
C2/IE stands for Command and Control Information Exchange
C2-protect stands for a defensive form of command and control warfare (US DoD)
C2-attack stands for an offensive form of command and control warfare (US DoD)
C1G2 stands for Class 1, Generation 2 (EPCglobal, RFID
C16 stands for Commodore 16 (home computer)