AP

AP most likely refers to:
 * Armor penetration — a game mechanic.
 * Assassin's Promise — an elite assassin skill.
 * An Assassin/Paragon player, e.g. the torch runner in a Shards of Orr speed clear.

Generally does not refer to these terms with the same acronym:
 * Apply Poison — a ranger skill.
 * Attribute points
 * Auspicious Parry — an elite warrior skill.