Tension-Cutting Laughter

Laughter that deflates the tension built up over a scene or sequence.

Laughter is a reaction normally associated with good humor, shared insight, and merriment being had. Other times, it signals that your are not going to get shivved in the back. Tension-Cutting Laughter is that second kind, the kind that says "Ah, so the Don doesn't think I'm a rat selling him out to the Feds" or "So Bob isn't going to kick my ass for insulting his daughter" or even simply "Mary does get I was kidding about selling her cat on eBay." It is akin to an exhale of relief after a particularly tense bit of drama has been wound up, the source of which is usually a misunderstanding.

Tension-Cutting Laughter is an efficient way for writers to build and release tension rhythmically throughout a narrative, and as such should be viewed as a Plot Device unrelated to whatever it is the characters are laughing about. The laughter could be in response to anything, whether it be the ridiculousness of the situation, something new and unrelated entering the scene, or even a joke. It could even simply be just laughter on its own, apropos of nothing. The point is it cuts the tension, allowing the audience to relax a little.

Can be a source of Mood Whiplash, doubly so if the Tension-Cutting Laughter is followed by the revelation that the one laughing actually is taking everything seriously.

This will often be found in stories employing Gallows Humor. May be the key ingredient in an "Everybody Laughs" Ending. Sometimes doubles as a Laugh with Me moment.

Examples of Tension-Cutting Laughter include:

Comic Book

In Johnny the Homicidal Maniac, there's a scene where Devi and Nny are on a date. Devi jokes about killing Nny and leaving his dead body. Both fall silent, but after a moment they both laugh.

Fan Works

Turnabout Storm: The massive laughter coming from the courtroom after Dash's Embarrassing Old Photo is shown almost makes forget the fact that she was going to be declared guilty just a few seconds ago. Of course, this being Ace Attorney, Mood Whiplash strikes again quickly.

Film

Done famously in Goodfellas after a prolonged scene where Tommy DeVito (Joe Pesci) appears to be offended at being called "funny" by Henry Hill (Ray Liotta), his rage visibly building over the course of several minutes. A bit of a twist in that it is Ray Liotta who breaks the tension.
Subverted in Pirates of the Caribbean: At World's End after they escape the Locker. Everybody pulls guns on everyone else, and after a minute they all start laughing... but then they remember that, yes, they do all have people to threaten.
Found at the beginning of Batman Returns. Max Shreck, a villain, and Selina Kyle, his secretary, break out in this after Max makes like he's going to choke her to death. After the laughter subsides, he pushes her out of the 45th floor window.
In The Hunt for Red October, when Jack Ryan, Captain Bart Mancuso and Seaman "Jonesy" Jones arrive aboard the titular sub, there is dead silence as the Americans and Russians stare at one another. But when Ryan accepts a cigarette from one of the Russian officers, he coughs while smoking (earlier, he'd explain that he doesn't smoke, repeatedly turning down cigarettes), which amuses the Russians and soon eases the tensions between both sides.

Literature

In Deryni Rising, though the protagonists are anticipating trouble from Charissa at Kelson's coronation, there's some larking about whilst dressing for the ceremony. Morgan strikes a pose in his finery and Duncan calls him conceited, wagging a finger in the role of scolding priest, whereupon both men burst out laughing, Morgan holding his sides and Duncan collapsing into a chair. The chapter's epigraph is "For surely laughter masks a nervous soul."
There are several occasions in Discworld novels when the narrator notes that characters are laughing at not-very-funny lines, because people who thought they were about to die will laugh at anything. In The Truth, Sacharissa's Tension-Cutting Laughter lasts too long, and William realises the tension isn't cut at all, and this is "the kind of laughter you die from".

Live-Action TV

In Breaking Bad, Tuco is the master of this.
On Farscape D'argo and John Crichton frequently did this.
Buffy lists all the ways the Scoobies have stuffed up since he left, ending with "And I'm sleeping with Spike." She then looks nervous as she waits for Giles to tear strips off her...only for Giles to burst into laughter. After a moment Buffy can't help but join him.