Watson's Biblical & Theological Dictionary 
The term era (not aera , as incorrectly written) is Spanish, signifying time, as in the phrase, de era en era, "from time to time." It was first used in the Era Hispanica, instituted B.C. 38, in honour of Augustus, when Spain was allotted to him in the distribution of the provinces among the second triumvirate, Augustus, Anthony, and Lepidus. It now usually denotes an indefinite series of years, beginning from some known epoch; and so differs from a period which is a definite series: as the era of the foundation of Rome, the era of the Olympiads, the era of Nabonassar, &c. See Epoch .
Webster's Dictionary 
(1): ( n.) A period of time reckoned from some particular date or epoch; a succession of years dating from some important event; as, the era of Alexander; the era of Christ, or the Christian era (see under Christian).
(2): ( n.) A fixed point of time, usually an epoch, from which a series of years is reckoned.
(3): ( n.) A period of time in which a new order of things prevails; a signal stage of history; an epoch.
Smith's Bible Dictionary 
Ge'ra. (A Grain). One of the "sons," that is, descendants, of Benjamin. Genesis 46:21. Gera, who is named, Judges 3:15, as the ancestor of Ehud, and in 2 Samuel 16:5, as the ancestor of Shimei who cursed David, is probably also the same person, (though some consider them different persons).
International Standard Bible Encyclopedia 
ē´ra : We find no definite era in use in Old Testament times, and such usage does not appear until we reach the period of the Maccabees. There are some references to important events that might have served as eras had they been generally accepted and constantly employed. Such was the Exodus; and this is referred to as the starting-point in fixing the date of the building of Solomon's temple ( 1 Kings 6:1 ), and also for the date of Aaron's death ( Numbers 33:38 ). An earthquake is referred to by Amos ( Amos 1:1 ) as a well-known event by which to date the beginning of his prophetic career; and Ezekiel in two passages refers to the captivity of Judah as a date for marking certain events in his life. Of these the Exodus would have been the most appropriate event to use as an era, since it marked the birth of the Hebrew nation; but the universal custom of antiquity was to date from the regnal years of the kings, as we see in the history of Egypt and Babylonia and Assyria; this custom was followed by the Israelites as soon as the kingdom was established, and was continued down to the Captivity. After the return of the Jews they naturally adopted the regnal years of the Persian kings, under whose rule they were, until the overthrow of the kingdom by Alexander. After this event, the era that prevailed most widely in Syria was that of the Seleucid kingdom, which began in 312 bc, and must have been familiar to the Jews, and we have evidence that they made use of it. When Simon the Maccabee secured the independence of the Jews from the Seleucid king, Demetrius II, in 141-140, they began to date their instruments and contracts from this event as is stated in 1 Macc 13:41, 42; and we find that the year of their independence is fixed by reference to the Seleucid era, the first year of Simon being the 170th of that era (see Josephus, Ant , Xiii , vi, 7). After this they used the era of Simon, dating by his regnal years; but whether they used this as a permanent era during the Asmonean Dynasty or dated simply from the accession of each king, we do not know. There is no doubt that the Seleucid era continued to be used throughout the country for several centuries after the downfall of the Seleucid kingdom, as we have abundant evidence from inscriptions. When the Romans took possession of Syria and Palestine, their era was of course employed by Roman officials, but this did not prevail among the people. The dynasty of the Herods sometimes employed their own regnal years and sometimes those of the emperors, as appears from their coins. The Jews must have been familiar with the eras employed by some of the Phoenician towns, such as Tyre and Sidon. Tyre had a local era which began in 126 bc, and Sidon one beginning in 112 bc; and most of the towns on the coast used the era of Alexander, dating from the battle of Issus, until the establishment of the Seleucid era. The Jews would be familiar with these from their commercial connections with the coast towns, but we do not know that they used them. They did not adopt the era of the Creation until after the time of Christ. It was fixed at 4,000 years before the destruction of the later temple, or 3760 bc.