Castrum was the term used for different sizes of camps including a large legionary fortress, smaller auxiliary forts, temporary encampments, and "marching" forts. The diminutive form castellum was used for fortlets, typically occupied by a detachment of a cohort or a century.
In English, the terms Roman fortress, Roman fort, and Roman camp are commonly used for castrum. However, scholastic convention tends toward the use of the words camp, marching camp, and fortress as a translation of castrum.
For a list of known castra see List of castra.
The term castrum appears in three Italic languages: Oscan, Umbrian and Latin. In Latin castrum was probably originally the term for an estate or a tract of land enclosed by a fence or a wooden or stone wall of some kind, as seems to be used in a few passages in Cornelius Nepos' works.
Considering that the earliest military shelters were tents made of hide or cloth, and all but the most permanent bases housed the men in barracks of tents placed in quadrangles and separated by numbered streets, a castrum may originally have been a tent.