(advanced music theory)
The term "pitch class" refers to a mapping of all pitches onto 12 distinct classes, roughly analagous to the typical "A B C D E F G" ordering of the pitches, but more precise and clear.
Typically, "C" is mapped to "0". All pitches one semitone above C (such as C#, Db, Ebbb, etc.) are mapped to "PC 1". All pitches two semitones above C are mapped to "PC 2", and so on.
Pitches are mapped to their corresponding pitch class independent of octave and regardless of spelling. Thus pitch classes achieve a simplicity of notation; no longer do we have to take a moment to recognize that E# is F is Gbb -- these are all "PC 5". Also, intervals can be found through the use of modular arithmetic in base 12; for example, the interval between PC 3 and PC 8 is five semitones, or IC 5, and between PC 9 and PC 4, the interval is seven semitones, also IC 5.