Ahh, I see. Dictionaries are rarely the best source for finding the meaning of concepts, particularly terms deeply rooted in the larger intellectual tradition. Atemporal is typically synonymous with eternal, but eternal traditionally doesn't have the meaning you may have thought. I'll explain. The theoretical/theological pedigree for both atemporal and eternal is tied to immutability. Immutable means changeless. Immutability has been considered primary for both grounding truth claims and for delineating the essential characteristics for any perfection since Parmenides and Plato. This is because what is perfect is complete and thus cannot change or be subject to any causative that would effect a change. Now, temporal is the adjectival referent for time. Time entails sequence and change as demonstrated by any time marker T and the movement of time through an attending T1,T2 T3 etc. Each instance T1, T2, T3 has its own beginning and end. This is why concepts like past, present and future are meaningful. Atemporality, per the prefix, is timelessness and thus precludes sequence. Eternal indicates constancy or duration extended independent of time. Thus, atemporality and eternity are often seen as synonymous and why the immutable is necessarily atemporal and eternal. This is how the terms are typically used with the Western Intellectual Tradition.
