Overview
The Roman Senate was the highest and constituting assembly of ancient Rome and its aristocracy. With different powers throughout its existence it lasted from the first days of the city of Rome (traditionally founded in 753 BC) as the Senate of the Roman Kingdom, to the Senate of the Roman Republic and Senate of the Roman Empire and eventually the Byzantine Senate of the Eastern Roman Empire, existing well into the post-classical era and middle ages.
During the days of the Roman Kingdom, most of the time the Senate was little more than an advisory council to the king, but being an electoral monarchy, it also elected new Roman kings. The last king of Rome, Lucius Tarquinius Superbus, was overthrown following a coup d'état led by Lucius Junius Brutus, who founded the Roman Republic.
During the early Republic, the Senate was politically weak, while the various executive Roman magistrates and later emperor which appointed the senators for life (or until expulsion by Roman censors) were quite powerful. Since the transition from monarchy to constitutional rule was most likely gradual, it took several generations before the Senate was able to assert itself over the executive magistrates. By the middle Republic, the Senate had reached the apex of its republican power. The late Republic saw a decline in the Senate's power, which began following the reforms of the tribunes Tiberius and Gaius Gracchus.
After the transition of the Republic into the Principate, the Senate lost much of its political power as well as its prestige. Following the constitutional reforms of Emperor Diocletian, the Senate became politically irrelevant. When the seat of government was transferred out of Rome, the Senate was reduced to a purely municipal body. That decline in status was reinforced when Constantine the Great created an additional senate in Constantinople.
From Wikipedia (CC BY-SA 4.0).