Capacity: What an entity may be or do - its potential, ability, power, talents etc.
The other nouns given in the definition are approximate synonyms, and the idea is basic and rather subtle. The fundamental problem with it, however it is called, is that what a thing is capable of may be much more than it, so far, has empirically shown, and this involves modal considerations, as also indicated by the 'may be'.
A fairly standard example of a capacity is that of an acorn to turn into an oak, in suitable conditions, but not into a butterfly or mermaid. This example indicates another strength or subtlety of capacities: That something may have the capacity to turn into another kind of thing, as also illustrated by the caterpillar and the butterfly.
Aristotle made much of capacities, though the term used in English translations tends to be 'potential'.
There is a related term, disposition, with which capacity must be contrasted: A capacity is something that a thing can do, in suitable conditions; a disposition is an organization of capacities,