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In Latin usage, the singular form castrum meant 'fort', while the plural form castra meant 'camp'. The singular and plural forms could refer in Latin to either a building or plot of land, used as a fortified military base.
In English usage, castrum commonly translates to "Roman fort", "Roman camp" and "Roman fortress". However, scholastic convention tends to translate castrum as "fort", "camp", "marching camp" or "fortress".
Romans used the term castrum for different sizes of camps - including large legionary fortresses, smaller forts for cohorts or for auxiliary forces, temporary encampments, and "marching" forts. The diminutive form castellum was used for fortlets, typically occupied by a detachment of a cohort or a centuria.
For a list of known castra, see List of castra.