Roman Senate
The council of ex-magistrates whose authority steered the Roman Republic for centuries, then survived as a dignified rubber stamp under the emperors.
The Roman Senate never actually ruled Rome, and that was the secret of its power. It could not pass a law, command an army in the field, or elect a magistrate. What it had instead was authority, auctoritas, the settled deference of a society that took its cues from a few hundred of its oldest, richest, and most experienced men. For roughly a thousand years, from the founding of the Republic in 509 BCE to the fall of the western empire, the Senate's advice was so heavy that magistrates ignored it at their peril.
Who sat there
The Senate was not elected. It was filled with ex-magistrates, men who had already served as quaestors, praetors, or consuls, and enrolled for life. That made it a permanent reservoir of governing experience in a Republic whose actual officials rotated out every single year. Its members came from a narrow aristocracy of a few hundred families; a new man like Cicero, who reached the consulship in 63 BCE without a single senator in his ancestry, was a rarity talked about for generations. Through this body ran the real business of the state: it managed the treasury, received foreign ambassadors, assigned commands and provinces, and oversaw the state religion. Its decrees carried the formula SPQR, Senatus Populusque Romanus, the Senate and People of Rome, stamped on standards and coins.
Power without the people
The People in that formula were mostly decoration. Rome was a republic, not a democracy: assemblies of citizens voted, but the system was weighted so heavily toward wealth and toward the Senate's guidance that ordinary Romans rarely set policy. The great convulsions of the late Republic, the Gracchi, Marius, Sulla, and Caesar, were in large part fights over whether the Senate's grip should be broken. Caesar's murder in 44 BCE was carried out by senators defending exactly that grip. They lost.
The dignified rubber stamp
Augustus understood that the way to hold absolute power in Rome was to leave the Senate standing. From 27 BCE he kept its ceremonies, its robes, its debates, and its decrees, while quietly controlling its membership, its finances, and every army that mattered. Senators still filled the top offices, and their decrees now carried the force of law, but a body that could not touch the legions was a body advising an emperor who did not need advice. Under later rulers the Senate became a court, a registry of honors, and a stage for flattery; Caligula's threat to make his horse a consul was a joke precisely because everyone knew the Senate would have done it.
The institution outlived Rome as an idea. Every modern senate, from Washington to Paris, borrows the name of a chamber whose founders meant it to be the sober, senior counterweight to the passions of the crowd, and which spent its last centuries proving how easily that counterweight becomes an ornament.