secondary dominant
A dominant chord that temporarily tonicises a scale degree other than the tonic
In Depth
A secondary dominant is labelled V/x, where x is the chord being tonicised. For example, V/V in C major is a D major chord, which is the dominant of G. These chords inject momentary chromatic colour without a full modulation, creating forward motion toward their target. Any diatonic chord except the diminished triad can serve as a temporary tonic. Secondary dominants are the most common source of accidentals in tonal music and form the foundation of applied chord theory.
The opening of Beethoven's First Symphony uses a chain of secondary dominants, which confused early audiences who could not immediately identify the home key.