Real Dictionary

gag
countable noun: A gag is a joke.
countable noun: A gag is something such as a piece of cloth that is tied around or put inside someone's mouth in order to stop them from speaking.
verb: If a person is gagged by someone in authority, they are prevented from expressing their opinion or from publishing certain information.
verb: If someone gags you, they tie a piece of cloth around your mouth or put a cloth inside it in order to stop you from speaking or shouting.
verb: If you gag, you cannot swallow and nearly vomit.

gag in American English
something put into or over the mouth to prevent talking, crying out, etc.
to cause to retch or choke
to retch or choke

gag in American English 1
intransitive verb: to retch or choke
noun: something put into a person's mouth to prevent speech, shouting, etc
transitive verb: to stop up the mouth of (a person) by putting something in it, thus preventing speech, shouts, etc

gag in American English 2
intransitive verb: to tell jokes or make amusing remarks
noun: a joke, esp. one introduced into a script or an actor's part
transitive verb: to introduce usually comic interpolations into (a script, an actor's part, or the like)

gag in American English 3
noun: a serranid game fish, Mycteroperca microlepsis, found along the southeastern coast of the U.S.

gag in British English 1
verb: to stop up (a person's mouth), esp with a piece of cloth, etc, to prevent him or her from speaking or crying out

gag in British English 2
noun: a joke or humorous story, esp one told by a professional comedian
verb: to tell jokes or funny stories, as comedians in nightclubs, etc