Definition and background:
A chord progression where the dominant chord is followed by the tonic chord ( V-I or V-i). The " V" represents the chord based on the fifth step of the scale and the " I" represents the chord based on the first step of the scale ("i" being the minor version of the chord based on the first step of the scale). The dominant to tonic progression ( V-I) represents the strongest sound of all cadences. In the tonality of "C" major, an authentic cadence would be the dominant ( V) G major chord ( G- B- D) moving to the tonic (I) C major chord (C- E- G). Also, Perfect Authentic Cadence, Final Cadence, Full Cadence, Full Close Cadence.A cadence that starts of the fifth of the key, and resolves to the tonic.
For more information on the music term "Authentic cadence" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary