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A YouTube Haiku, according to the subreddit (a subsection of popular social news site Reddit) that coined the name, is a video that lasts no longer than 14 seconds, and is, as they describe it, pure poetry.
Cheap humor isn't what they are looking for. The subreddit's welcome message states "Videos of kids falling down aren't exactly poetic (we don't delete them, but they may be downvoted)."
In layman’s terms, they are YouTube one-liners, funny enough to make you laugh in a very short amount of time.
The subreddit does allow videos longer than 14 seconds, but considers them poetry, not haiku. At the moment, a large portion of the top videos on the page are labeled as poetry, which the above gallery reflects.