A tribus, or tribe, was a division of the Roman people, constituting the voting units of a legislative assembly of the Roman Republic. The word is probably derived from tribuere, to divide or distribute; the traditional derivation from tres, three, is doubtful.
According to tradition, the first three tribes were established by Romulus; each was divided into ten curiae, or wards, which were the voting units of the comitia curiata. Although the curiae continued throughout Roman history, the three original tribes that they constituted gradually vanished from history.
Perhaps influenced by the original division of the people into tribes, as well as the number of thirty wards, Servius Tullius established thirty new tribes, which later constituted the comitia tributa. This number was reduced to twenty at the beginning of the Roman Republic; but as the Roman population and its territory grew, fifteen additional tribes were enrolled, the last in 241 BC.
All Roman citizens were enrolled in one of these tribes, through which they were entitled to vote on the election of certain magistrates, religious officials, judicial decisions in certain suits affecting the plebs, and pass resolutions on various proposals made by the tribunes of the plebs and the higher magistrates. Although the comitia tributa lost most of its legislative functions under the Empire, enrollment in a tribe remained an important part of Roman citizenship until at least the third century AD.