An annulment is the cancellation of revocation of something, like a marriage. After an annulment, it's like the marriage never happened.
The root nul (from Latin nullum) of this word is a good clue to its meaning: the underlying verb annul originally meant "reduce to nothing." An annulment also reduces something to nothing by canceling or terminating it, usually in a legal sense. The most common use of the term is the annulment of a marriage, which not only ends the marriage, but legally makes it as if the marriage never happened. Annulment is like a legal eraser.