gag

a joke that is repeated or referred to multiple times throughout a story or performance

a joke that has been told many times before

a joke whose punchline is very easy to guess

a joke based on something disgusting or offensive

a joke on a particular subject

to tie someone's hands and feet and cover their mouth to prevent them from moving or speaking

to place a piece of cloth over someone's mouth to prevent them from speaking

to create a new joke, especially for a performance