Hilarity is loud, uncontrollable laughter or a situation that causes it. When something strikes people as so funny that they can't help bursting into laughter, that's hilarity. Picture a classroom where someone's presentation goes hilariously wrong: maybe their poster falls down repeatedly, their prop breaks in an unexpected way, and they accidentally say “organism” instead of “organization.” The resulting hilarity might have everyone doubled over laughing, including the presenter.

The word suggests boisterous and infectious laughter: one person's laughter sets off another's until the whole group is caught up in it. When a comedy show causes hilarity, audience members might be wiping tears from their eyes or clutching their sides.

You might hear someone describe something as hilarious, meaning extremely funny. If your friend tells a story about their dog stealing an entire pizza off the counter, that's hilarious. The phrase “hilarity ensued” is a common way to describe what happened after something funny occurred, like when a squirrel ran through an outdoor assembly and everyone scrambled in different directions.