equal temperament

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Modification of the intervals of just intonation in the tuning of instruments of fixed intonation to permit harmonic modulation. It allows one to modulate to or play in any of the 12 major or 12 minor keys.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the division of the scale based on an octave that is divided into twelve exactly equal semitones