The Complete Idiot''s Guide to Music Theory

larghetto
Tempo marking for a slow tempo, slightly faster than largo.
largo
Tempo marking for a slow and dignified tempo.
lead sheet
A piece of sheet music that contains a single staff for the melody,
with the accompanying chords written above the staff.
leading tone
The note that is a half step below the tonic; thus leads up to the
tonic.
lento
Tempo marking for a slow tempo.
Locrian mode
A (theoretical) mode starting on the seventh degree of the
corresponding major scale. 
See mode.
Lydian mode
A mode starting on the fourth degree of the corresponding
major scale. 
See mode.
major
The most common mode, consisting of the following intervals: whole-
whole-half-whole-whole-whole-half.
major chord
A chord with a major third (1-3-5).
major seventh chord
A major triad with a major seventh added (1-3-5-7).
measure
A group of beats, indicated by the placement of bar lines on the
staff.
mediant
The third degree of a scale, or a chord built on that degree (III).
melody
The combination of tone and rhythm in a logical sequence.
meter
The organization of beats and their divisions.
mezzo forte
Medium loud. (Abbreviated as 
mf.
)
mezzo piano
Medium soft. (Abbreviated as 
mp.
)
microtone
An interval smaller than a semitone, prevalent in some non-
Western music and twentieth-century art music.
minor
One of three modes, each with a flatted third of the scale. Natural minor
is identical to 
Aeolian mode
, with the following intervals: whole-half-whole-whole-
half-whole-whole. Harmonic minor contains the following intervals: whole-
half-whole-whole-half-whole-half-half. Melodic minor is different ascending
and descending; ascending, the intervals are whole-half-whole-whole-whole-
whole-half, whereas descending it uses the same intervals as natural minor.