12 equal temperament (12-ET) is the musical system that divides the octave into 12 parts, all of which are equally tempered on a logarithmic scale, with a ratio equal to the 12th root of 2. That resulting smallest interval, 1⁄12 the width of an octave, is called a semitone or half step.
Prince Zhu Zaiyu constructed 12 string equal temperament tuning instrument, front and back view
Zhu Zaiyu's equal temperament pitch pipes
Simon Stevin's Van de Spiegheling der singconst c. 1605.
An equal temperament is a musical temperament or tuning system that approximates just intervals by dividing an octave into steps such that the ratio of the frequencies of any adjacent pair of notes is the same. This system yields pitch steps perceived as equal in size, due to the logarithmic changes in pitch frequency.
Zhu Zaiyu's equal temperament pitch pipes
Figure 1: The regular diatonic tunings continuum, which include many notable "equal temperament" tunings.