In Scholastic philosophy, the aevum (also called aeviternity) is the mode of existence experienced by angels and by the saints in heaven. In some ways, it is a state that logically lies between the eternity (timelessness) of God and the temporal experience of material beings. It is sometimes referred to as “improper eternity”. The word aevum is Latin, originally signifying “age”, “aeon”, or “everlasting time”; the word aeviternity comes from the Medieval Latin neologism aeviternitas.
The concept of the aevum dates back at least to Albertus Magnus’s treatise De quattuor coaequaevis. Its most familiar description is found in the Summa theologica of Thomas Aquinas. Aquinas identifies the aevum as the measure of the existence of beings that “recede less from permanence of being, forasmuch as their being neither consists in change, nor is the subject of change; nevertheless they have change annexed to them either actually, or potentially.” As examples, he cites the heavenly bodies (which, in medieval science, were considered changeless in their nature, though variable in their position) and the angels, which “have an unchangeable being as regards their nature with changeableness as regards choice”.
Frank Sheed, in his book Theology and Sanity, said that the aevum is also the measure of existence for the saints in heaven: “Aeviternity is the proper sphere of every created spirit, and therefore of the human soul... At death, [the body’s] distracting relation to matter’s time ceases to affect the soul, so that it can experience its proper aeviternity.” 
- Anzulewicz, Henryk. "Aeternitas - aevum - tempus. The Concept of Time in the System of Albert the Great". The Medieval Concept of Time: Studies on the Scholastic Debate and Its Reception in Early Modern Philosophy. Ed. Pasquale Porro. Leiden: Brill Academic Publishers, 2001.
- Charlton T. Lewis and Charles Short, A Latin Dictionary (Oxford, Clarendon Press, 1879), 64-65.
- Albertus Magnus, De IV coaequaevis, tract. 2, qu. 3.
- Thomas Aquinas, Summa Theologica, First Part, Question 10, Article 5. 
- Sheed, Frank. Theology and Sanity. 2nd ed. San Francisco: Ignatius Press, 1993.