Alternate Advancement (AA) is a system developed and introduced with the Rise of the Godslayer expansion that allows you to advance your character beyond the level cap.
The system is available for use beginning at level 20, but is mainly designed for use at level 80 (everything you do level 20 and beyond counts, but you make no significant gains until level 80).
There are 3 types of AA: Mastery, Expertise, and Prowess. Mastery experience is gained by doing PVE, Prowess through PVP, and Expertise can be used for either and is gained from pots for doing special tasks.