2. The Gr. adjective aionios is derived from the noun aiōn, and bears the basic meaning, in consequence, of “belonging to time in its duration,” i.e., constant, abiding, eternal (Biblio-Theological Lexicon of New Testament Greek, Cremer, pp. 78, 79; also s.v. aiōn, pp. 74-79). It was found in this meaning in classical Gr.: e.g., Plato, Rep. 2:363D, “the fairest reward of virtue being, in their estimation, an everlasting carousal.” Biblical and ecclesiastical Gr. used the word commonly, the LXX, for example, as the standard tr. of the Heb. עוֹלָם, H6409.

The frequency of its use in Johannine contexts is notable. The significance mingles future and present, for “eternal life” in Christian belief is not only a life of endless duration, but a quality of life in which the possessor shares by faith in God’s eternal being.

In his careful essay, More New Testament Words (pp. 24-32), William Barclay examines the word aionios in classical and NT contexts, with full analysis of the meaning and its spiritual applications. Eternal life, as he points out in conclusion, were it a mere duration, could become the burden which Tithonus, in the deeply meaningful Gr. myth, found it to be. “Life is only of value when it is nothing less than the life of God—and that is the meaning of eternal life.” It necessarily follows that it has no termination.

3. A rare word in NT contexts, but common enough at all stages of classical Gr. from Homer onwards, is aidios. It occurs in Romans 1:20 and Jude 6. See Life; Immortality.

International Standard Bible Encyclopedia (1915)

(`olam; aionios, from aion):

The word "eternal" is of very varying import, both in the Scriptures and out of them.

1. `Olam:

In the Old Testament, the Hebrew word `olam is used for "eternity," sometimes in the sense of unlimited duration, sometimes in the sense of a cycle or an age, and sometimes, in later Hebrew, in the signification of world. The Hebrew `olam has, for its proper nodetitle equivalent, aion, as signifying either time of particular duration, or the unending duration of time in general. Only, the Hebrew term primarily signified unlimited time, and only in a secondary sense represented a definite or specific period. Both the Hebrew and the Greek terms signify the world itself, as it moves in time.

2. Aion, Aionios:

In the New Testament, aion and aionios are often used with the meaning "eternal," in the predominant sense of futurity. The word aion primarily signifies time, in the sense of age or generation; it also comes to denote all that exists under time- conditions; and, finally, superimposed upon the temporal is an ethical use, relative to the world’s course. Thus aion may be said to mean the subtle informing spirit of the world or cosmos--the totality of things. By Plato, in his Timaeus, aion was used of the eternal Being, whose counterpart, in the sense-world, is Time. To Aristotle, in speaking of the world, aion is the ultimate principle which, in itself, sums up all existence.. In the New Testament, aion is found combined with prepositions in nearly three score and ten instances, where the idea of unlimited duration appears to be meant. This is the usual method of expressing eternity in the Septuagint also. The aionios of 2Co 4:18 must be eternal, in a temporal use or reference, else the antithesis would be gone.

3. Aidios:

In Ro 1:20 the word aidios is used of Divine action and rendered in the King James Version "eternal" (the Revised Version (British and American) "everlasting"), the only other place in the New Testament where the word occurs being Jude 1:6, where the rendering is "everlasting," which accords with classical usage. But the presence of the idea of eternal in these passages does not impair the fact that aion and aionios are, in their natural and obvious connotation, the usual New Testament words for expressing the idea of eternal, and this holds strikingly true of the Septuagint usage also. For, from the idea of aeonian life, there is no reason to suppose the notion of duration excluded. The word aionios is sometimes used in the futurist signification, but often also, in the New Testament, it is concerned rather with the quality, than with the quantity or duration, of life. By the continual attachment of aionios to life, in this conception of the spiritual or Divine life in man, the aeonian conception was saved from becoming sterile.

4. Enlargement of Idea:

In the use of aion and aionios there is evidenced a certain enlarging or advancing import till they come so to express the high and complex fact of the Divine life in man. In Greek, aiones signifies ages, or periods or dispensations. The aiones of Heb 1:2, and 11:3, is, however, to be taken as used in the concrete sense of "the worlds," and not "the ages," the world so taken meaning the totality of things in their course or flow.

5. Eternal Life:

Our Lord decisively set the element of time in abeyance, and took His stand upon the fact and quality of life--life endless by its own nature. Of that eternal life He is Himself the guarantee--"Because I live, ye shall live also" (Joh 14:19). Therefore said Augustine, "Join thyself to the eternal God, and thou wilt be eternal."

See Eternity.