- C4 (butane)
- C-4 (Composition C-4 plastic explosive)
- C4AH (Center for Advanced Hindsight)
- C4CM (Command, Control, Communications, and Computer Counter-Measures)
- C4D (Cinema 4D)
- C4I (Command, Control, Communications, Computers, and Intelligence)
- C4IFTW (Command, Control, Communications, Computers, and Intelligence For The Warrior)
- C4IM (Command, Control, Communications, Computers, and Information Management)
- C4I or C4I (Command, Control, Communications, Computers, and Intelligence)
- C4IP2 (Command, Control, Communications, Computers, and Intelligence Power Projection)
- C4IR (Command Control Communication Computers Intelligence And Recognition)
- C4ISR (Command, Control, Communications, Computers, Intelligence, Surveillance, and Reconnaissance)
- C4ISR or C4ISR (Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance)
- C4ISTAR (Command, control, communications, computers, intelligence, surveillance, target acquisition, reconnaissance)
- C4ME (Command, Control, Communications, and Computers Mission Environment)
- C4 or C4 (Command, Control, Communications, and Computers)
- C4P (National Center for Policy Research for Women and Families)
- C4S (Command, Control, Communications, and Computer Systems)
- C-5 (Cargo aircraft model 5 (Galaxy))
- C5 (Command, Control, Communications, Computers, and Counter-intelligence)
- C6 (hexanes)
- C7+ (heavy hydrocarbon components)
- Ca (Carcinoma)