Measure - A group of beats indicated by the placement of bar lines on the staff. The space between two bar lines.

Melody - In general, a succession of musical tones. It represents the linear or horizontal aspect of music.

Mezzo - Half, Medium.

Mezzo forte - Medium loud.

Mezzo piano - Medium soft.

Middle C - The note C in the middle of the Grand staff, and near the middle of the piano.

Moderato - Moderate speed.

Music - The organization of sounds with some degree of rhythm, melody, and harmony.

Music Theory - The study of how music is put together.