Both these words are given the sense of immortal, by lexicographers. Liddell and Scott, standard authorities, give it thus. Plutarch uses aphthartos as incorruptible, immortal. And it seems to be the exact word corresponding to our words, incorruptible [not liable to corrupt, or to be corrupted], and immortal [not mortal—not subject or liable to death].
Athanasia, while it is properly translated immortal according to usage, does not so much have the sense of not liable to die, as that of unchangeability. Hence aphthartos is the word which most closely corresponds to our word immortal, i.e., not mortal, not perishable, not corruptible. This is shown by the relationship between corruptible and incorruptible in 1 Cor. 15:53 and 54, which in the Greek as in the English stand related, of the same root, the Greek being phthartos and aphthartos. Not so, however, the words mortal and immortal in the same verses. In the English these words are closely related, but the Greek uses words totally distinct and not related—thnetos and athanasia, the sense being, "This mortal [dying condition] shall put on [or assume] immortality" [a lasting or unchangeable condition].
So, then, the attempt of some to make out that incorruptible refers to one state, and immortality to another, is without foundation, and probably the result of lack of thoroughness in the examination of the subject. Prof. Young, Liddell and Scott, and all translators are right in using the two English words immortal and incorruptible interchangeably. As above suggested, however, we would have preferred it had athanasia been translated unchangeability in the three cases where it occurs, although our word immortality covers the idea of unchangeability.
With this change 1 Cor. 15:53,54 would read thus:—"The [special] dead [i.e., the saints] will be raised incorruptible [i.e., immortal, not liable to corrupt, decay, or perish] and we [of the same special class] shall be changed." "For of necessity this corruptible [diseased, perishable condition] must be invested with incorruptibility [imperishable quality] and this mortal [dying condition] must be invested with immortality" [unchangeability]. "And when this corruptible [perishable condition] shall be invested with incorruptibility [imperishable quality] and this mortal [dying condition] shall be clothed with unchangeability [immortality], then will that prophetic promise be fulfilled [which says] Death will be swallowed up in victory." That is to say: when this special class, the dead and we, the overcomers, the saints, are changed to undying, changeless conditions, then will that prophecy of Isaiah 25:8 begin to be fulfilled to the world—the Millennial work of abolishing death and restoring life will then go on successfully.