Roman Consul

In 509 BCE, Lucius Junius Brutus and  the Roman army expelled the tyrant king Tarquin the Proud from Rome, bringing the Roman monarchy to a halt. And then the Roman Republic was born.

The Romans did not want to give power to a single person, fearing that it would be abused, as it had been by the monarchy.  Instead, every year, the Roman citizens elected two consuls. Each of them alternately held power for one month.

The Consul, or the Imperator of the Roman army, presided over the Senate. Initially, only patricians could become consul, but starting from 360 BCE, the position was open to plebeians. The consul was required to be above 41 years of age. During the time of war, the consul with military skill was chosen.   Initially they had a lot of power, but later the censorial powers were transferred to the censor and judicial powers were also taken away from them. The consul position was held for only one year at a time, and the consul could only be reelected after 10 years. This rule changed after Lucius Cornelius Sulla, who was consul twice in a row.  When their term as consul ended, they may become the proconsul (governor) for one of the Roman provinces.

During a war, consuls were granted dictatorship or imperium power. After the war, the consuls had to give up the power, but Julius Caesar refused. This led to the fall of the Roman Republic.
Under the Roman Empire, Consuls did not have much power as they previously did.

Reference
Roman Consul