The Complete Idiot''s Guide to Music Theory

compound chord
Two chords sounded together. Typically notated with a
vertical slash between the two chords.
concert pitch
The actual (non-transposed) pitch of a piece of music;
some instruments (such as the trumpet) read their music transposed from
concert pitch.
consonance
Harmonious combination of tones. The opposite of 
dissonance.
contralto
A lower subset of the alto voice, even lower than the traditional
alto range; not used in all choral music.
contrapuntal
See counterpoint.
contrary motion
Where one voice moves in the opposite direction to
another.
counterpoint
Two or more simultaneous, independent lines or voices.
Some music theorists apply strict rules to the creation of contrapuntal lines.
crescendo
Gradually louder.
cut time
The 2/2 time signature.
D.C. al Coda
Navigation marking meaning to go back to the beginning
and play to the Coda sign; then skip to the Coda section.
D.C. al Fine
Navigation marking meaning to go back to the beginning
and play through to the end.
D.S. al Coda
Navigation marking meaning to go back to the Segno sign
and play to the Coda sign; then skip to the Coda section.
D.S. al Fine
Navigation marking meaning to go back to the Segno sign
and play through to the end.
decrescendo
Gradually softer.
diatonic
Notes or chords that are in the underlying key or scale. For
example, in the key of C Major, the diatonic notes are C, D, E, F, G, A, and
B; all other notes are 
chromatic
.
diminished chord
A chord with a minor third and a diminished fifth 
(1-
3-
5).
dissonance
A combination of tones that sounds discordant and unstable,
in need of resolution to a more pleasing and stable harmony. The opposite
of 
consonance.