Dyad
A dyad (less commonly, diad) is a chord of two pitch classes. In its simplest voicing, a dyad contains two notes, but it may also contain octave doublings (assuming octave equivalence). For example, a power chord is a dyad because it contains only two pitch classes a perfect fifth apart, even though it usually contains at least three notes.
Dyads can be classified by the interval between the notes, and as such the words dyad and interval are sometimes used interchangeably.