Pronounced \ə-ˈnəl\, annul is a transitive verb. A transitive verb must have a direct subject and object. For example, "Tom wanted to annul his marriage with Suzanne." The subject is Tom and the object is marriage.
The inflected forms for annul are annulled and annulling. It's synonym (different word with similar meaning) is nullify. For example, "The hard-won goal was nullified by a penalty."
The first definition is to, "reduce to nothing : obliterate." For example, "The planet was annulled by the impact of the meteor."
The second definition of annul is to, "make ineffective or inoperative : neutralize." For example, "The shock collar annulled the dog's barking."
The third meaning of annul is, "to declare or make legally invalid or void." For example, "The court ordered the marriage annulled because it was never consummated."
Encyclopedia Brittanica defines annulled as the "legal invalidation of a marriage." It is different from a dissolution or divorce because it says the the marriage was never valid. In order to receive an annulment the marriage must be flawed. It must be proven that, for example, one member was underage, or incompetent, mentally unstable, or failed to disclose a prior marriage.