In music, a unison ( ) is an interval, the ratio of 1:1 or 0 half steps and zero cents. Two tones in unison are considered to be the same pitch, but are still perceivable as coming from separate sources. The unison is considered the most consonant interval while the near unison is considered the most dissonant. The unison is also the easiest interval to tune.
The unison is abbreviated as P1.
A pair of tones in unison can have different "colors" (timbres), i.e. come from different musical instruments or human voices. Voices with different colors have, as sound waves, different waveforms. These waveforms have the same fundamental frequency but differ only in the amplitudes of their higher harmonics.
When several people sing together, as in a chorus, the simplest way for them to sing is to sing in "one voice", in unison. If there is an instrument accompanying them, then the instrument must play the same notes being sung by the singers (in order for there to be unison). Otherwise the instrument is considered a separate "voice" and there is no unison. If there is no instrument, then the singing is said to be a cappella. Music in which all the notes sung are in unison is called monophonic.
From this sense can be derived another, figurative, sense: if several people do something "in unison" it means they do it simultaneously, in tandem, in lockstep. Related terms are "univocal" and "unanimous".
Monophony could also conceivably include more than one voice which do not sing in unison but whose pitches move in parallel, always maintaining the same interval of an octave. A pair of notes sung one or a multiple of an octave apart are almost in unison, due to octave equivalency.
When there are two or more voices singing different notes, this is called "part singing". If they are singing notes at different pitches but with the same rhythm this is called homophony. An example is a barbershop quartet or a choir singing a hymn. If each voice is singing an independent line (either the same melody at a different time, or different melodies) this is called polyphony.
On synthesizers, the term unison is used to describe two or more oscillators that are slightly detuned in correspondence to each other, which makes the sound fatter. This technique is so popular that some modern virtual analog synthesisers have a special oscillator type called "super saw" or "hyper saw" that generates several detuned sawtooth waves simultaneously. This method is often used in techno and trance.