The dominant in music is the fifth note of a major or minor scale. In a C scale, the fifth and dominant note will be G. Chords built on this note are called dominant chords. In the key of C, the dominant chord will be G-B-D. Keys have the same relationship as notes, so the dominant key will be the key of the fifth note. So in C, the key of G is the dominant key. In musical theory, the dominant is written with the roman numeral "V".