M.M.: Originally "Mälzel Metronome" but now metronome marking.
Maggiore: The major mode.
Major Second: A second, or musical interval covering two adjacent staff positions, spanning two semitones.
March: Music for marching, such as in a parade or procession.
Mixolydian Mode: A medieval mode whose scale pattern is that of playing G to G on the white keys of a piano.
Meter Signature: See time signature.
Modal: Pertaining to modes.
Modern: Music written in the 20th century, or contemporary music.
Modulation: 1. To change keys, the movement from one tonic center to another.
Monody: A solo or unison song with accompaniement.
Monothematic: Music based upon a single theme.
Monophony: Music written in a single melodic line, as opposed to polyphony.
Morceau: "Morsel". A musical work or composition.
Mordent: An ornament consisting of a single alternation between a given pitch, and the one immeditatly below or above it--called an inverted mordent.
Motet: A choral composition, usually on a religious text.
Motif: A short musical idea, or melodic theme that runs through a piece.
Musicology: The study of music and music history.
Music Drama: Opera, specifically that of Richard Wagner and his successors.
Musique Concrete: Music composed by manipulating aucoustically generated recorded real-world sounds.