Perfect Authentic Cadences (PAC) are harmonic progressions that create a strong sense of resolution, typically moving from the dominant chord (V) to the tonic chord (I) in root position. The defining characteristic of a PAC is that both chords must be in root position, and the highest note of the tonic chord must be the tonic itself. This type of cadence is often used to signal the end of a phrase or section in music, providing a satisfying closure.