Wiktionary
n. The act or process of respecifying; a change from a previous specification
Wikipedia
Respecification (to respec or to respecify) in many MMORPG titles such as Star Wars Galaxies, City of Heroes, and City of Villains allows a player to change the attributes of his/her character.
A player's character gains differing abilities, characteristics, buffs, armor, powers, weapons and combat actions as the player/character advances through the levels of the game. Sometimes a player may realize that a certain power is no longer useful to their character, may want to get a power that is more fun to use/play, or even find that a certain choice earlier in the game has now restricted the player from powers or choices that they would like to explore. By respecing, players can save valuable game time by revamping their character to gain certain powers that may be otherwise unobtainable, or to revive a character that may have been considered unplayable.
While the phrase "respec" is used in the World of Warcraft MMO to mean the same thing, the game's creator, Blizzard Entertainment, uses the word "specialization," so that to respec is to respecialize. In Champions Online respecing is called retconning because of the comic theme of the game.