Temperament
A temperament is a tuning system that slightly compromises the pure intervals of just intonation (or more generally, of any tuning) to meet other requirements.
In regular temperament theory, the term temperament usually refers to a regular temperament. Besides that, there are irregular well temperaments, though analysis of well temperaments as specific tunings of regular temperaments is possible.
History
In Western music history, the first temperament to be described by theorists was the meantone temperament, in the beginning of the 16th century. It was developed in an attempt to solve some of the issues of the Pythagorean tuning, the most commonly used tuning system until then. In particular, the pure fifths (3/2) are slightly compromised in order to obtain either pure major thirds (5/4), in quarter-comma meantone, or slightly compromised major thirds in other meantone tunings. The linear structure of meantone has laid the foundations for the development of regular temperament theory.
Because 12-tone meantone (meantone[12]) has its own issues, a new kind of temperaments appeared in the 17th century: well temperaments. These have an irregular structure that allow for more flexibility in the way each note is tuned, and thus how each interval is compromised. As a result, most scales sound different when they are transposed to a different key.