The unison (interval ratio 1/1) is the interval between two tones that are identical in pitch. In the harmonic series, 1/1 is the 1st harmonic, and likewise in the subharmonic series 1/1 is the first subharmonic- this is because it acts as the fundamental to both series.
Measured in cents (or any other logarithmic measure such as millioctaves, EDO steps, etc.), the unison's size is exactly 0. This is because the distance between two identical pitches is zero. As such, the unison can be considered as a degenerate interval.
In just intonation, 1/1 represents the base frequency from which an interval is measured.