In music theory, unison is a special interval because it is a null interval: Two music notes with the same pitch form a unison.
What does unison look like?
Here is an example of a melodic unison (two notes of a melody) and a harmonic unison (two notes in a chord):
Unison can only be perfect, it cannot be major, minor, diminished, augmented, (and so on...) because by definition unison represents an interval formed by two notes of the same pitch .
Inversion of the perfect unison
The inversion of the perfect unison is the perfect octave.
Here is an example of a perfect octave:
Musical examples of perfect unison
Beethoven's Moonlight Sonata
After the introduction composed of arpeggios, the Beethoven's Moonlight Sonata begins with a succession of unison:
Interval identification game
You will find this interval in my Intervals identification game: