The consul was an elected Roman magistrate, with regal power during the Republican period of Roman history.
Following the expulsion of the kings of Rome, Rome became a Republic, with a new type of leader. For this purpose, the Romans invented the new position of consul (by 181 B.C., limited to men of at least 43 years of age). It conferred a limited term of absolute power; however, the power wielded was less than that of the king, since it was split between 2 men (consuls) and limited to a single year. Ten years were supposed to elapse before serving as consul a second time.
The people, via the Comitia Centuriata annually elected these 2 consuls during the Republic. During the Principate, the Senate appointed multiple consuls. Originally, the consuls came from the patricians. Later, they were chosen from either patrician or plebeian, and later, there was a requirement that at least one consul be plebeian.
Consuls were responsible for war, justice, and finance. Later, subordinate magistrates, like the quaestors, took over some of the consul's functions and power. Each consul could negate the other and was supposed to heed the advice of the Senate. The consul could be tried for misdeeds after his single-year term in office.
A consul held imperium, had control of the army, and, following the end of his term in office, could look forward to governing a province as proconsul -- a position that tended to be lucrative. Julius Caesar, as proconsul, waged military campaigns in Gaul.
The position of consul continued during the imperial period, when emperor was the highest office, but the power and term in office of the consul decreased, as his number increased.
- William Smith Dictionary of Greek and Roman Antiquities