This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Laughter was a physical reaction in sentient beings for expressing mirth, satisfaction, or derision. The act of making laughter was called laughing.
In humanoids, laughter was a movement of certain muscles of the face, especially around the mouth, resulting in a lighting up of the face and eyes. It was accompanied by the emission of explosive or chuckling sounds from the chest and throat.
In 3189, the USS Discovery computer suggested that one way Captain Saru could heal the crew was through laughter. According to the computer, sentient beings found laughter to be both healing and meaningful. It then recommended the 20th century comedians Buster Keaton and Charlie Chaplin, who were known as "communal unifiers without the burden of language".
Later that day, the computer showed the Buster Keaton movie "Sherlock Jr." in the ship's shuttlebay for the crew. Watching the film, they were healed and unified through laughter. (DIS: "Forget Me Not")