A silent panel in sequential art. Usually the next-to-last panel in a serialized comic strip, since it approximates the comedic pause before a punchline.
Particularly efficient comic artists may copy and paste adjacent panels, since the point of the Beat Panel is usually that the characters are frozen in contemplation. Another variation is to have two beat panels, with just a quizzical change of expression in the second to show a character's confusion (more likely to happen in a four-panel strip than a three-panel strip). It can also be unusually long to indicate a long beat.
Compare Silent Scenery Panel. A Beat is the (un)spoken version.
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Anime and Manga
The manga of Azumanga Daioh uses these a lot, often at the end of the comic when the gag is a lack of action. These usually translate to (often hilarious) stretches of awkward silence in the anime. One of the more memorable ones:
Panel 1: Chiyo: Oh, Sakaki, you're here already! It'll be an hour before the others get here! Panel 2: Sakaki: It's okay... I'll wait outside... with Mr. Tadakichi... Panels 3 and 4: (The same image of Sakaki, perfectly content, sitting under a tree with Mr. Tadakichi)
An example of how they translate this: in the anime, during the same scene, the camera stays focused on Sakaki as every other main character walks into Chiyo's house.
Hentai manga Witchcraft uses this to great comedic effect. Kagami is trying to get Kaoru to relax so she can effectively hypnotize him (Mildly NSFW text):
Kagami: Well, the easiest way is that relaxed state right after ejaculation. All right, ejaculate. Kaoru: Right... (Beat Panel) Kaoru: ... ... Ejaculate? Kagami: Yes. Kaoru: You mean where it ... squirts out? Kagami: That's right. Now hurry up.
Ayase: I never said I approved (of you choosing Kanale). In fact there's another problem before all this: Kanale already belongs to me.
... The silence goes on for multiple panels.
There's one early on in the Battle Royale manga. Shuya asks Noriko how she can trust him so easily. She says "You didn't peek at my panties." Cue the ellipsis and possibly the only funny moment in the story.
Lone Wolf and Cub probably sets a record. The last chapter contains the same Beat Paneleight times across multiple pages. It underscores how dramatic the moment is; the implication is that the moment was practically endless for all watching.
The Keith Giffen/J. M. DeMatteis comedy incarnation of Justice League used this all the time, sometimes featuring entire Beat Pages.
J.M. DeMatteis's run on Spectacular Spider-Man featured a beat page - but it wasn't funny, rather it was one of the creepiest pages ever seen in a comic book.
Used excessively in Invincible, then Lampshaded when the main character gets his comics signed by an artist who comments on his use of copying and pasting panels.
In the comic Teen Titans, after Beast Boy asked Raven to go with him on a "not-a-date", there was a beat panel before Raven said "Let's go".
In an Invincible Iron Man issue, Pepper Potts admits to Maria Hill that she slept with Tony. A shocked Maria Hill admit that she also slept with Tony a few days before Potts. Follows a succession of panel with both looking shocked, each at each other, and then each looking down, visibly angry. After that, Hill mutters a simple "Tony Stark. Tony fucking Stark.
Peter David enjoys regular use of these. The Madrox mini-series contained a number of examples.
In Scott Pilgrim, Scott asks Wallace what the website for Amazon.ca is. Wallace gets his beat panel with a dumbfounded look and a series of ellipses and responds ".... Amazon.ca".
Life in Hell occasionally uses these to an extreme. Matt Groening refers to these as "all those Akbar and Jeff strips where they stare at each other." Keep in mind there were often dozens of panels to a page.
During the Joss Whedon run on Astonishing X-Men, the morning after Peter Rasputin and Kitty Pryde finally make love, they meet with Wolverine in the kitchen. Two beat panels follow; one where Wolverine looks at Peter, and one where he looks at Kitty. He then returns to his breakfast, muttering, "'Bout time."
Pretty much every comic has used this at some point. It's been around since the early days of comics, but it really took off in the 60's and 70's, when a new generation of cartoonists raised on films and television sought to make their comics more cinematic. Doonesbury is often credited with popularizing the beat, and it remains one of the most frequent users of this trope.
A comic strip by David Lynch was almost entirely made up of beat panels. The same ones. For ten years.
Interestingly, The Far Side occasionally pulled this off in a one-panel strip. The visual was some awkward situation, while the punchline came in the caption.
Gary: "Oh, I'm terribly sorry! I didn't mean to be rude, it's just I used to think you didn't want to go on a second date with me because I was too geeky." 'beat panel' Nanase: "Yes, it was entirely because I was a lesbian" Gary:"Phew! Well, ain't that a boost for my fragile ego!"
Arguably, this page is compromised entirely of beat panels. Which makes them ten in a row, when counting the last panel of the previous page too.
Kat: Scarlet?! What are you wearing? Scarlet:Soopa soots. Kat: You built them just now? Amber: Nuh-uh. We've been working on them since the giant bug incident. (beat) Kat and Art:What giant bug incident?! Amber: Oooh...
Stickman and Cube does this often, usually when one of the characters does or says something incredibly bone-headed.
Ruben Bolling's Tom the Dancing Bug mocked this trope mercilessly, with a strip that claimed that "the more silent reaction panels before the punchline, the funnier it is!", demonstrating by showing a strip with one beat panel, two beat panels and 100 beat panels (which was, in fact, hilarious).
Fans! recently used beat panels(and other silent panels) in an innovative way in the arc "Crossover", which involves an attack on a crossword-fans convention. Each of the 18 pages has six square frames, with periodic beat panels, and each beat panel is framed in a thick black outline. As the final page displays, when all the panels are arranged in order, they form a crossword panel, with each beat panel as a black space, and the first letter in each of the other panels is used in the crossword solution.
Homestuck proper finally gets four beat panels (a lot for a comic that usually has one panel on each page) when Gamzee prototypes Equius into Dirk's sprite and causes a highly-anticipated awkward moment.
This webcomic  consists entirely of beat panels in the first comic. But what do you expect from a comic called "The Mind-numbingly Boring Webcomic"? It appears to get better and actually funny in the next actual comic, but who knows how it will proceed.