In ancient Rome, any of various assemblies of the people, which could meet only when summoned by a magistrate.
The earliest citizen assembly was the comitia curiata, which later survived only for routine, formal functions. The comitia centuriata, of military origin, was the principal assembly 486–287 BC and was open to all citizens. The comitia tributa, also open to all citizens, first appeared 357 BC. From 287 BC its legislative functions gradually increased and by the later republic it was, along with the concilium plebis, the chief legislative assembly.
Under the emperor Augustus and his successors, all these assemblies lost their independence and they functioned merely to give official authorization to imperial decisions.
Comitia curiata The earliest assembly was the comitia curiata. It was based on the ancient curiae, 30 in all, ten each of the original three kinship tribes of Servius Tullius, sixth king of Rome (traditionally ruled 578–534 BC). Its importance had decreased by the time of the early republic, but it continued to function for certain religious, traditional, and formal purposes. In Cicero's day, it was regularly represented by a random collection of magistrates' attendants and its activities were quite uncontroversial.
Comitia centuriata Rome's citizens were liable for military service and its legions divided into ‘hundreds’ or centuriae. But the comitia centuriata early became separate from the military organization, and evolved into Rome's major legislative assembly. There were 193 centuries in all, arranged in census (property) classes. The wealthiest citizens of the first class were enrolled in 70 centuries, and together with the 18 ‘equestrian’ centuries constituted just under half of the total number (votes were counted by block votes of centuries, not by individual voters) while the poorest citizens (capite censi or proletarii) were all enrolled into a single century which voted last. Hence, the comitia centuriata, which was presided over by consuls or praetors (whom it elected), was inevitably heavily weighted in favour of the better-off citizens. But from the middle years of the republic it ceased to be a legislative assembly (save in rare instances), its chief importance being its function of electing censors, consuls, and praetors, though it did very occasionally exercise its power to try capital cases of treason.
Comitia tributa The original three ‘kinship’ tribes were early superseded by a division into ‘tribes’ based on locality and residence (four ‘urban’ tribes and a number of ‘rural’ tribes which eventually totalled an additional 31 as Rome's territory expanded in Italy). These 35 tribes formed the comitia tributa (here again votes were counted by tribes, not by individual voters). From the early 3rd century BC this assembly superseded the comitia centuriata as the main legislative assembly of Rome. (It was more ‘democratic’, since each individual member of a tribe had one vote, regardless of his wealth or property.) It was summoned by consuls or praetors, but could not discuss the business they brought before it, only vote yes or no to the proposals they presented to it (usually only after the presenters had themselves secured the prior agreement of the Senate).
Concilium plebis Open only to plebeians, the concilium plebis was effectively the comitia tributa minus the patrician members. Originally created when the tribum plebis came into existence as the ‘people's champions’ against the patrician oligarchy in the early 5th century BC, the concilium plebis elected and was presided over by the tribunes of the plebs. After a long struggle a law (the Lex Hortensia) was passed 287 BC, under which the resolutions (formally known as plebiscita) of the concilium plebis took on the full force of law, on a par with those of the comitia tributa, binding on the whole Roman state.
Thus, in the late republic the comitia tributa and the concilium plebis were the two major legislative assemblies, and virtually identical save that the latter excluded the (by now relatively few) patrician citizens. But, while the comitia tributa was summoned and presented with its business by a consul or a praetor, this function was performed for the concilium plebis by a tribune of the plebs. Hence very often (although not always) the concilium plebis (as notably under the presidency of the tribunes Tiberius and Gaius Gracchus) was more the vehicle of ‘reform’ legislation than was the comitia tributa under its older and more ‘established’ consul presidents.