Acta means ‘the things that have been done’ and has two specialized, overlapping senses in Roman history; one is a gazette, the other is official acts, especially of an emperor.
The Acta diurna were a gazette, whose publication dates from before 59 bce (a 2nd-cent. bce example of these is quoted by Renaissance antiquarians but its authenticity has been doubted); from the late republic onwards it recorded not only official events and ceremonies, but lawsuits and public speeches, and was read both at Rome and in the provinces (Asc. 30–1 C; Tac. Ann. 16. 22). The Acta senatus (or Commentarii senatus) constituted the official record of proceedings in the senate, first published in 59 bce (Suet. Iul.20). Under the Principate a senator was selected by the emperor to be responsible for the record (Tac. Ann. 5. 4). The proceedings were available to senators but Augustus forbade their wider publication (Suet. Aug.36). Tacitus used, or depended on, authorities who used both the Acta senatus (e.g. Ann. 15. 74) and the Acta diurna (Ann. 3. 3.).
The exact definition of acta meaning enactments was not easy. The term was eventually held to cover the ‘constitutions of emperors’ (i.e. edicta, decreta, and rescripta, etc., see constitutions). Under the Roman republic magistrates took an oath, on entering office, to respect the laws of the state. The first recorded use of an oath to observe acta was that taken by all the magistrates to observe the acta of Caesar in 45 bce (App. BCiv. 2. 106). When his acta were ratified by the senate after his murder (Cic. Phil. 1. 16 ff.), the looseness of the term gave scope for confusion and corruption. Oaths were taken to observe the acta of Augustus in 29 and in 24 bce. (Cass. Dio 51. 20, 53. 28). In order to equate the emperor's enactments with other sources of law, magistrates, emperors included (Cass. Dio 60. 10), took the oath to observe the acta of previous emperors, except those whose acta, directly after their death, were explicitly rescinded (rescissio actorum) or at least excluded from the oath (Plin. Ep. 10. 58), though the wise enactments of even bad emperors might survive their death (Gai. Inst. 1. 33; Dig. 48. 3. 2). However, the relation of the acta of the living emperor to those of his predecessors raised problems. At first moderate emperors, such as Tiberius and Claudius, sought to restrict the oath to the acta of Divus Augustus, excluding their own acta (Suet. Tib. 67; Tac. Ann. 1. 72; Cass. Dio 60. 10), but with the increase of autocracy, the oath came to include the acta of reigning emperors.