A castellum in Latin is usually:
- a small Roman fortlet or tower, a diminutive of ('military camp'), often used as a watchtower or signal station like on Hadrian's Wall. It is distinct from a , which is a later Latin term that was used particularly in the Germanic provinces.
- a distribution, header, and settling tank for municipal water in a Roman aqueduct (known as a castellum aquae/castellum divisorium).
It is the source of the English word "castle".