(1):

(n.) A value that is nearly but not exactly correct.

(2):

(n.) A continual approach or coming nearer to a result; as, to solve an equation by approximation.

(3):

(n.) An approach to a correct estimate, calculation, or conception, or to a given quantity, quality, etc.

(4):

(n.) The act of approximating; a drawing, advancing or being near; approach; also, the result of approximating.