noun.
The determination of the limits or outlines of a thing; a marking out; the state of being clearly marked out or outlined; specifically, in optics, the defining power of a lens—that is, its ability to give a clear, distinct image of an object in all its important details. This depends upon the freedom of the lens from spherical and chromatic aberration.
noun.
The act of stating the signification of a word or phrase, or the essential properties of a thing.
noun.
A statement of the signification of a word or phrase, or of what is essential to the conception of any given thing; an explanation of how any given kind is distinguished from all other kinds.