Douglas Harper's Etymology Dictionary
"being by itself," 1690s, from Medieval Latin aseitas "state of being by itself," from Latin a "from" + se "oneself" (see suicide).
Wiktionary
n. (context theology metaphysics English) Being self-derived, in contrast to being derived from or dependent on another; being self-existent, having independent existence.
Wikipedia
Aseity (from Latin a "from" and se "self", plus -ity) refers to the property by which a being exists in and of itself, from itself, or exists as so-and-such of and from itself. The word is often used to refer to the Christian belief that God contains within himself the cause of himself, is the first cause, or rather is simply uncaused, though many Jewish and Muslim theologians have also believed God to be independent in this way. Notions of aseity as the highest principle go back at least to Plato and have been in wide circulation since Augustine, though the use of the word 'aseity' began only in the Middle Ages.
Usage examples of "aseity".
He is Spiritual, for were He composed of physical parts, some other power would have to combine them into the total, and his aseity would thus be contradicted.
For if He began in time, He would need a prior cause, and that would contradict his aseity.