A / aera
noun

aera

gen. aerae · gender feminine · decl. 1st
[from aera
counters;
counters; v. aes, 2. E., later Lat.
a given number; according to which a reckoning; calculation is to be made
In math., a given number, according to which a reckoning or calculation is to be made, Vitruvius (Vetrubius) Rufus ap. Salmas. Exercc. I. p. 483.—
Anitem of an account
Anitem of an account (for the class. aera, plur. of aes, Ruf. Fest. in Breviar. init. The passage of Lucil. cited by Non. 2, 42, aera perversa, is also prob. plur.).—
An era; epoch from which time is reckoned
An era or epoch from which time is reckoned, Isid. Orig. 5, 36; cf. Inscr. Orell. II. p. 374.