(1):

(n.) Sounding alone.

(2):

(n.) Sounded alike in pitch; unisonant; unisonous; as, unison passages, in which two or more parts unite in coincident sound.

(3):

(n.) A single, unvaried.

(4):

(n.) Harmony; agreement; concord; union.

(5):

(n.) Identity in pitch; coincidence of sounds proceeding from an equality in the number of vibrations made in a given time by two or more sonorous bodies. Parts played or sung in octaves are also said to be in unison, or in octaves.