Punch line

A punch line is the final part of a joke

Joke

A joke is a short story or ironic depiction of a situation communicated with the intent of being humorous. These jokes will normally have a punchline that will end the sentence to make it humorous. A joke can also be a single phrase or statement that employs sarcasm...

 or comedy sketch, usually the word, sentence or exchange of sentences which is intended to be funny and to provoke laughter

Laughter

Laughter is an audible expression or appearance of happiness, or an inward feeling of joy . It may ensue from jokes, tickling, and other stimuli. Inhaling nitrous oxide can also induce laughter; other drugs, such as cannabis, can also induce episodes of strong laughter...

 from listeners.

For instance, in the following well-known joke:

A man walks into a bar with a duck under his arm.

The bartender asks: "Say, where did you find the pig?"

"It's not a pig, it's a duck," the man answers.

To which the barman replies: "I was talking to the duck."

Punch lines generally derive their humor from being unexpected.

A punch line is the final part of a joke

Joke

A joke is a short story or ironic depiction of a situation communicated with the intent of being humorous. These jokes will normally have a punchline that will end the sentence to make it humorous. A joke can also be a single phrase or statement that employs sarcasm...

 or comedy sketch, usually the word, sentence or exchange of sentences which is intended to be funny and to provoke laughter

Laughter

Laughter is an audible expression or appearance of happiness, or an inward feeling of joy . It may ensue from jokes, tickling, and other stimuli. Inhaling nitrous oxide can also induce laughter; other drugs, such as cannabis, can also induce episodes of strong laughter...

 from listeners.

For instance, in the following well-known joke:

A man walks into a bar with a duck under his arm.

The bartender asks: "Say, where did you find the pig?"

"It's not a pig, it's a duck," the man answers.

To which the barman replies: "I was talking to the duck."

Punch lines generally derive their humor from being unexpected. "Punch line" is probably an Americanism

American English

American English is a set of dialects of the English language used mostly in the United States. Approximately two thirds of native speakers of English live in the United States.English is the most common language in the United States...

, but the "punch" could be related to biting lines delivered by the "Punch" character in Punch and Judy

Punch and Judy

Punch and Judy is a traditional, popular English puppet show featuring the characters of Punch and his wife Judy. The performance consists of a sequence of short scenes, each depicting an interaction between two characters, most typically the anarchic Punch and one other character...

 shows. In previous centuries, a joke was sometimes a "bite" or a "hit.", in Italian it is still called a "battuta" (= blow).

The classic stand-up

Stand-up comedy

Stand-up comedy is a style of comedy where a comedian performs for a live audience, usually speaking directly to them. It is usually performed by a single comedian with the aid of a microphone, either hand-held or mounted on a stand...

 punch line sound is a sting

Sting (percussion)

A sting is a short sequence played by a drummer to punctuate a joke, especially an obvious pun. A sting is often used as accompaniment during cabaret- or circus-style shows...

 (erroneously called a rimshot

Rimshot

A rimshot is the sound produced by hitting the rim and the head of a drum at once, with a drum stick. Rimshots are usually played to produce a more accented note, and are typically played loudly. However, soft rim shots are possible....

) on drum

Drum

The drum is a member of the percussion group of music instruments, technically classified as a membranophone.. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a shell and struck, either directly with parts of a player's body, or with some sort of...

s.
Not all jokes rely on their punchlines for laughs. Shaggy dog stories are long-winded jokes where the punchline is deliberately anticlimactic, and are not intended to elicit laughter.