The dominant scale degree, known as the fifth scale degree of a diatonic scale, plays a crucial role in tonal music as it creates tension that seeks resolution to the tonic. This degree, often represented as 'V', is essential for establishing the harmonic function within a key, and its chords are commonly used to lead back to the tonic. The dominant scale degree is vital in creating a sense of movement and direction in music, particularly through various chord progressions and cadences.