"Treason" is, of course, an English word. Usually it is used to translate maiestas, which it does a poor job of translating--better is the French lèse-majesté. Maiestas is derived from the comparative form of magnus, maior (maius in the neuter), and means therefore something like "superiority." A pater familias, for example, maintained maiestas over his household. It's legal use is actually an abbreviation of terms like maiestam minutio, "decreasing of superiority." Violation of maiestas was, at least if the possessor was important enough, a capital offense. Maiestas was not limited to people. It extended to the state cults, magistrates, and even the populus itself. Thus the violation of a father's authority was contrary to his maiestas, as were certain types of desecration of state religious sites. But "treason" is mainly used to translate specifically crimen maiestatis adversus populum Romanum vel securitatem eius, "the crime of [claims to] superiority over the Roman people or their security," or maiestas minuta Romani populi, "the diminished superiority of the Roman people." The precise requirements of the charge changed over time, but it was first instituted under the lex Appuleia in 103. A Sullan lex de maiestate wrote in more explicit rules, establishing legal limits for provincial promagistrates, and Caesar established banishment as the usual punishment. The vagueness of the charge continued to be an issue; maiestas was often leveled in the Republic against failure to comply against comitial or tribunician decisions. The establishment of the Principate expanded the charge of maiestas greatly, by extending it to cover actions seen to be against the emperor in some way. Augustus leveled it against conspirators, and even ensnared Julia's lovers under the law. Tacitus complains that Tiberius extended the law to include malicious speech and writing against the emperor. The precise wording of the law was never changed, though. The Digest describes maiestas as acting against the people by planning conspiracy against a magistrate or the state in the former of murder or revolt (planned or actually carried out), collusion with an enemy, etc. Nothing explicit about the emperor, but the political realities of the Principate meant that the emperors could easily eleven the charge against political opponents. The Digest also describes a couple other forms of maiestas, consisting of preventing a magistrate from fulfilling his duties, even the desecration of public iconography.

There were a couple other forms of treasonable activity (proditio, the crime of collusion with an enemy, or perduellio, turning hostile to the state, which was poorly-defined and all but died out in the late Republic), but these soon were replaced by the various forms of maiestas