Castra (pl.) is a Latin term used during the Roman Republic and Roman Empire for a military 'camp', and castrum (sg.) for a 'fort'.

In current English use, the peculiarity of the noun having different meanings in the singular and plural is sometimes less rigorously observed, given that both meanings indicate fortified positions used by the Roman army.

A castrum was the fortified base of a Roman legion, a detachment thereof, or of auxiliary units, providing secure locations for training, administration, and defense. The army used a variety of fortified positions, both in size and function, ranging from temporary marching camps (castra aestiva) to large, permanent fortresses (castra stativa) that housed entire legions. They were typically designed with a standardized layout, including a rectangular plan, defensive walls, gates, and internal streets arranged in a grid pattern, reflecting Roman military discipline and engineering expertise. Permanent castra often became the foundations for towns and cities across the Roman Empire, many of which still bear traces of their Roman origins in their modern layouts and names. These fortifications played a crucial role in the expansion and maintenance of Roman power, enabling the army to project control over vast territories and respond quickly to threats.

Castrum appears in Oscan and Umbrian, two other Italic languages, suggesting an origin at least as old as Proto-Italic. Julius Pokorny traces a probable derivation from *k̂es-, 'cut', in *k̂es-tro-m, 'cutting tool'. These Italic reflexes based on *kastrom include Oscan castrous (genitive case) and Umbrian castruo, kastruvuf (accusative case). They have the same meaning, says Pokorny, as Latin fundus, 'estate', 'tract of land', referring to a prepared or cultivated tract, such as a farm enclosed by a fence or wall. Cornelius Nepos uses Latin castrum in that sense: when Alcibiades deserts to the Persians, Pharnabazus gives him an estate (castrum) worth 500 talents in tax revenues. This is a change of meaning from the reflexes in other languages, which still mean some sort of knife, axe, or spear. Pokorny explains it as "a camp, as a cut-off piece of land".

If this is the civilian interpretation, the military version must be "military reservation", a piece of land cut off from the common land around it and modified for military use. All castra must be defended by works, often no more than a stockade, for which the soldiers carried stakes, and a ditch. The castra could be prepared under attack within a hollow square or behind a battle line. Considering that the earliest military shelters were tents made of hide or cloth, and all but the most permanent bases housed the men in tents placed in quadrangles and separated by numbered streets, one castrum may well have acquired the connotation of tent.

The commonest Latin syntagmata (here phrases) for the term castra are:

In Latin the term castrum is much more frequently used as a proper name for geographical locations: e.g., Castrum Album, Castrum Inui, Castrum Novum, Castrum Truentinum, Castrum Vergium. The plural was also used as a place name, as Castra Cornelia, and from this comes the Welsh place name prefix caer- (e.g. Caerleon and Caerwent) and English suffixes -caster and -chester (e.g. Winchester and Lancaster). Castrorum Filius, "son of the camps", was one of the names used by the emperor Caligula and then also by other emperors.^{[citation needed]}

Castro, also derived from Castrum, is a common Spanish family name as well as toponym in Spain and other Hispanophone countries, Italy, and the Balkans, either by itself or in various compounds such as the World Heritage Site of Gjirokastër (earlier Argurokastro). In Greek, writers used the terms stratopedon ('army camp') and phrourion ('fortification') to translate castrum and castellum, respectively.^{[citation needed]}