A squib is something small. What specifically is small depends on the context.
31. I will not detonate Squibs.
—from 35 More Things I Am Not Allowed To Do At Hogwarts, by Nightspore
Small explosives used to simulate gunshot wounds in movies and TV. Usually paired with a "blood bag"—which is exactly what it sounds like, a small plastic sac filled with a liquid that looks like blood—to produce what supposedly looks like a "real" wound. According to most actors who've had to use them, they hurt.
Like most pyrotechnics, squibs will not work if they get too wet. In addition to its literal meaning, the term "damp squib" is sometimes used as a metaphor for a setup that fails in execution.
Short pieces, used as filler in magazines.
A literary squib is usually witty or satirical. It can be similar to a lampoon, but does not need to parody a particular work.
A linguistic squib is both a very short and a scholarly article. According to The Other Wiki, "A squib may outline anomalous data but not suggest a solution, or develop a minor theoretical argument." Compare this sort of squib with a "snippet" (which is essentially a stand-alone footnote).
"Squib" also has a meaning as a verb, meaning to hit weakly. Most commonly heard in sports broadcasts, both American football (a purposely weaker kickoff in order to prevent the return team to set up its normal return and/or force someone other than its normal returner (usually someone bigger and slower than the normal returner) to field it, and baseball (a ball hit so softly that by the time it gets to any of the fielders, the batter is already at first base.)
Not to be confused with:
- Non-magical children of wizard parents in the Harry Potter universe, which are termed "squibs" in-world.
- The foxlike species whose hat is trading, from the Star Wars tabletop games
- Squibs (1921 film), starring Betty Balfour
- Squibs (1935 film), a remake of the 1921 film, also starring Balfour
- A Squab, which is a pigeon.