A Roman legion (from Latin legio "military levy, conscription," from legere — "to choose") normally indicates the basic ancient Roman army unit recruited specifically from Roman citizens. The organization of legions varied greatly over time but were typically composed of up to 5,000 soldiers, originally divided into maniples and later into cohorts. Maniples/Cohorts were divided into centuries.
In reference to the early Kingdom of Rome (as opposed to the republic or empire) "the legion" means the entire Roman army.
For most of the Roman Imperial period, the legions were a part of the Imperial army and formed its elite heavy infantry, recruited exclusively from Roman citizens (provincials who aspired to citizenship gained it when honorably discharged from the auxiliaries). Each legion always included a small cavalry attachment. The Roman army (for most of the Imperial period) consisted mostly of "auxiliary" cohorts who provided additional infantry, and the vast majority of the Roman army's cavalry.
Because of the enormous military successes of the Roman Republic and the Roman Empire, the legion has long been regarded as the prime ancient model for military efficiency and ability.
See List of Roman legions for a catalogue of known late republic, early Empire and late Empire legions, with dates in existence, emblem and locations of deployment.