equal temperament

Short definition from Wiktionary

Noun

1. 1
   A tuning system in which every interval (as a ratio of frequencies) is some exactly positive integral power of some smallest, other than the unison, base interval. In particular
2. 2
   A tuning system in which every interval (as a ratio of frequencies) is some exactly positive integral power of some smallest, other than the unison, base interval. In particular: Twelve-tone equal temperament: The European semi-harmonic interval compromise among the chromatic notes.