The Probert Encyclopaedia of Music

CADENCE

In music, cadence is the name given to the closing - usually last two - chords of a phrase. The varieties of cadence may be grouped as perfect, imperfect and interrupted. The perfect must have its last chord on the tonic. When the penultimate chord is on the subdominant it is called an 'authentic'; when on the dominant, a 'plagal' cadence. The harmony of the imperfect is often that of the perfect reversed. The interrupted is a progression of chords leading the ear to expect a tonic chord, but another is substituted for the latter; the effect is often as charming as it is unexpected.
Research Cadence