Equivalence
Equivalence is the phenomenon of psychoacoustics that two notes an octave apart are considered "(substantially) equivalent" to the brain. The question of whether it can (and should) generalize or generalizes to other intervals is still being studied. This question relates to the height of the interval's denominator in the (odd) harmonic series and another phenomenon presumably of psycho-acoustics that melodies within the range of a tenth are considered to sound "cohesive" or "coherent" to the brain. If equivalence can be perceived for other intervals, the best candidates are those of very high consonance, such as 3/1, the next harmonic after the octave.
The set of all equivalent pitches is a pitch class.