(sounded as 1; It. unisono). 1. A TONE of the same PITCH as a given tone; also, a higher or lower OCTAVE of the given tone. Some texts use the term prime. 2.Performing the same exact pitches on a POLYPHONIC instrument or with two or more voices and/or MONOPHONIC instruments. The description is often applied to playing octaves or even double octaves. 3. In the piano, a group of two or three strings tuned to the same pitch and struck by one hammer; a string in such a group is called a unison string.