In his etymologies (Book 5, Chapter 38, sec. 4) Isidore of Seville defined Aeternum as follows:
Nam aevum est aetas perpetua, cuius neque initium neque extremum noscitur, quod Graeci vocant αἰῶνας; quod aliquando apud eos pro saeculo, aliquando pro aeterno ponitur. Unde et apud Latinos est derivatum.
Given my limited knowledge of Latin, I would translate that as:
The eon, avum, is an uniterrupted age, whose beginning and end is unknown. The Greeks call them aions, eons, a term they sometimes use for saeculum, sometimes for aeternum; Whence the Latins derrived it.
(In other words, a long periode of indefinite duration.)
Comments Welcome–is that the substance of what Isidore wrote, or is my translation in error?