Interval

English Wikipedia has an article on:
English Wikipedia has an article on:

An interval (or dyad; less commonly, diad) is a chord of two different notes.

The main property of an interval is its size, defined as the difference in pitch between its two notes. Since pitch perception is logarithmic, an interval can be described with a frequency ratio or a logarithmic measure of that ratio, such as cents. Intervals are usually named after their size, which might explain why some dictionaries define the term interval as the size itself.

A rational interval is an interval whose frequency ratio is a rational number. Its logarithmic measure is then necessarily irrational[1]. A tuning system based exclusively on rational intervals is said to be in just intonation. Conversely, an irrational interval is an interval whose frequency ratio is an irrational number. In that case, however, its logarithmic measure may or may not be rational. An interval with a rational logarithmic measure is always irrational, but some intervals have both irrational ratios and logarithmic measures.

Another property is harmonic entropy, a measure of concordance, which is usually associated with consonance and dissonance.