Temperament

a strict set of frequency relationships..the music of the past 30 years has been firmly rooted in the 12-note chromatic, equal-tempered scale fully explored by Johann Sebastian Bach in the 18th century.

source: http://www.soundonsound.com/sos/jul03/articles/synthsecrets51.asp

The process of setting the correct spacing between the notes (on an instrument) is known as setting the temperament. In an equal tempered scale, chords can be shifted at will without harmful effect.

source: http://www.precisionstrobe.com/apps/pianotemp/temper.html

or:

In music, the altering of certain intervals from their acoustically correct values to provide a system of tuning whereby music can move from key to key without unacceptably impure sonorities. It is particularly necessary for keyboard instruments, the pitches of which cannot be varied in performance. Many systems have been devised, going back to the late 15th cent. "Just Intonation" refers to systems in which some fifths are tuned unacceptably small so that others may be pure. "Temperament" refers to systems that distribute the impurities throughout the tuning. Of these, "Equal Temperament" divides the octave into 12 equal half-steps, leaving all intervals except the octave slightly impure.

source: http://encyclopedia2.thefreedictionary.com/Temperament