Pitch class
A pitch class is a set (equivalence class) of all pitches that are a whole number of octaves (2/1) apart, e.g., the pitch class C consists of the Cs in all octaves. Thus the pitch class "C" is the set
[math]\displaystyle{ \left\lbrace \ldots, C_{-2}, C_{-1}, C_0, C_1, C_2, \ldots \right\rbrace }[/math]
In terms of frequencies expressed in Hertz, assuming a base frequency for middle C of 262 Hz, this would be {…, 65.5, 131, 262, 524, 1028, …}. In terms of MIDI note numbers, we can write it as {…, 36, 48, 60, 72, 84, …}.
In a nonoctave xenharmonic system, an interval other than the octave might be used to define equivalence. For example, in Bohlen–Pierce tuning and other equal divisions per tritave, all pitches separated by a whole number of tritaves (3/1) may be considered equivalent.