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Respec is generally an MMO term, and is short for Re-specification. Basically, it means being able to undo or rethink many or all of the decisions you've made concerning a character. You can switch out powers or abilities for replacements, or change a focus from one area to another.
Depending on the game there may be a cost involved, such as monetary or completion of some large task. There may also be a limit on the number of times a respec can be done.