Tribune: Roman official whose task it was to protect the people against oppression.
In the fifth century BCE, the republican magistracies were monopolized by aristocrats called patricians. This caused great tensions with two other groups: the poor, who had to appeal to a patrician judge against arbitrary decisions by patrician magistrates (e.g., consuls), and the rich non-patricians, who were supposed to fight in battle and to pay taxes, but were not allowed a vote in the decision about war and peace or to oversee the spending of their money.
In c.490 BCE (or, to use the Varronian chronology, which is too often confused with our era, 494), these two groups united and demanded political rights. From now on, the opposition was called the plebs. They created the 'anti-magistracy' of the tribunus plebis, who was to defend the rights of the non-patricians. In a lex sacrata (sacred law), the plebeians swore that they would defend the tribune's person at all costs, which made him sacrosanct (i.e., he could not be attacked by the patrician magistrates). This enabled him to veto (forbid) measures by consuls, sentences by praetors and financial decisions by quaestors. After a brief struggle, the patricians recognized the two (later ten) tribunes, but demanded that they would not veto military decisions. The tribunes were therefore some sort of anti-magistrates elected by the people's assembly (consilium plebis).
The tribunes were also chairmen of the people's assembly, superseded the election of the aediles, and had to assist poor plebeians. In the fourth and third centuries, when the conflict between the patricians and plebeians was over, new functions were added: for example, the tribunes could convene a Senate meeting and arrest magistrates. Their houses had to remain open for visitors even during the night, and they were not allowed to be more than a days' journey from Rome.
Although the tribunes had a potentially very revolutionary function, they were usually moderates - at least after the aims of the plebeians had been reached in the 360s, when the patrician offices were opened for plebeians (in 366, according to an inaccurate chronology). However, in the second century, the magistracy became revolutionary again, when Tiberius Sempronius Gracchus used it to propose an agrarian law to the people's assembly against the will of the Senate (133).
This caused many changes in the tribune's office. Firstly, one of his colleagues, Marcus Octavius, vetoed the proposal: something that had never happened before. As a result, Gracchus deposed Octavius - again, something that was unheard-of. To make sure that the agrarian law was executed, Gracchus wanted to be reelected - something that had not happened in two centuries. After this incident, the responsibilities and functions of the tribune were varying.
Ultimately, Tiberius Sempronius Gracchus was murdered by conservative senators. Later, his brother Gaius suffered the same fate. The lesson was that social and political reforms could only take place when a tribune controlled the people's assembly and had military backing. In the late second and first half of the first century, several tribunes tried to do this, but they usually failed (e.g., Lucius Appuleius Saturninus in 100). Later, reformers were looking for successful military commanders who 'bought' tribunes (e.g., Pompey and Julius Caesar).
When the emperor Augustus (27 BCE - 14 CE) founded the monarchy, he accepted permanent rights as tribune (tribunicia potestas) and counted his regnal years in this fashion (e.g., 'in the twelfth year of his tribunicial powers'). The office had become powerless because the people's assembly had disappeared; nonetheless, it still existed and politicians would serve one year as tribunes (usually when they were 27 years old).