Tonality In Western music since the seventeenth century, the pitches of the scale are organized so that they are felt to gravitate toward one principle pitch -- the tonic -- which is also the first note of the scale. This first or tonic pitch gives its name to the key.
Tonic harmony A harmony that is built on a tonic, i.e., the first scale degree or starting pitch of a diatonic scale.
Dominant harmony A harmony that is built on the dominant, i.e., the fifth scale degree of a diatonic scale.
Diminished chord A diminished chord consists of three or four pitches a minor third apart, resulting in an ambiguous harmony that could modulate to several keys.
Key In tonal music, the pitch relationships that establish a single pitch as a tonal center or tonic (or key note), with respect to which the remaining pitches have subordinate functions. There are two types or modes of keys, major or minor, and any of the twelve pitches can serve as a tonic. There are thus in principle 24 different keys.
Modulation In tonal music, the process of changing from one key to another, or the result of such change. Tonal plan The sequence of keys and modulations in a specific piece of tonal music.