Encyclopedias, Almanacs, Transcripts, and Maps
- Clay-Hard Pan
- Clayoquot Sound
- Clean Air Act (1963, 1970, 1990)
- Clean Coal Technology
- Clean Water Act (1972, 1977, 1987)
- Clear-Cutting
- cleavage, rock
- Clemen, Carl
- Clement of Rome
- Clements, Frederic E. (1874 – 1945) American Ecologist
- click language
- client-server
- Climate Change and Global Warming
- Climatic Effects
- Climax (Ecological)
- Clitoridectomy
- Clod
- Clothing: Clothing and Religion in the East
- Clothing: Clothing and Religion in the West
- Clothing: Dress and Religion in America's Sectarian Communities