In music theory, the term mode or modus is used in a number of distinct senses, depending on context.
Its most common use may be described as a type of musical scale coupled with a set of characteristic melodic and harmonic behaviors. It is applied to major and minor keys as well as the seven diatonic modes (including the former as Ionian and Aeolian) which are defined by their starting note or tonic. (Olivier Messiaen's modes of limited transposition are strictly a scale type.) Related to the diatonic modes are the eight church modes or Gregorian modes, in which authentic and plagal forms of scales are distinguished by ambitus and tenor or reciting tone. Although both diatonic and Gregorian modes borrow terminology from ancient Greece, the Greek tonoi do not otherwise resemble their medieval/modern counterparts.
In the Middle Ages the term modus was used to describe both intervals and rhythm. Modal rhythm was an essential feature of the modal notation system of the Notre-Dame school at the turn of the 12th century. In the mensural notation that emerged later, modus specifies the subdivision of the longa.
Outside of Western classical music, "mode" is sometimes used to embrace similar concepts such as Octoechos, maqam, pathet etc. (see § Analogues in different musical traditions below).