Interval
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.
An interval is rational if its ratio is a rational number, and its logarithmic measure is then necessarily irrational. Conversely, an interval is irrational if its 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.