That which is capable of existence; its synonyms are thing, something.
There are two uses of the term: the participial use (see existence), and the substantival, the definition of which is the one here given.
The term being in the substantival sense is applicable to anything that either actually exists or can exist, for being is either actual, i.e., existent, or merely possible.
It is contrasted with absolute nothingness, such as the impossible, e.g., a square circle, rather than with the merely non-existent.
This term stands for the simplest of all our concepts, viz., the mere capacity for existence, and is the widest in application since it represents substances, accidents, modes of existence, God, and creature.
Briefly whatever is not absolutely nothing is something or being.
New Catholic Dictionary