If a phrase ends with a dominant-to-tonic harmonic progression, we call that progression an authentic cadence.
A perfect authentic cadence (PAC) signals the greatest degree of melodic and harmonic repose: it occurs when the bass progression ^5 - ^1 harmonizes a stepwise motion to the tonic (^2 - ^1 or ^7 - ^1) in the soprano voice. It is analogous to a period at the end of a sentence.
Any other V - I cadence will signal a lesser degree of repose and is called an imperfect authentic cadence (IAC). Most commonly, IACs retain the bass sol - do pattern but come to rest on a scale degree other than ^1 in the soprano.
