A lot of Twitter conventions and jargon—perhaps most—have come from users rather than from the company. The language around the service is no exception, and "tweet" is a perfect example.
A term created by users, "tweet" refers to a single Twitter post. The term is also sometimes used as a verb, as in, "We're live-tweeting the four-hour wait at Pizzeria Bianco."
Trivia: Twitter itself didn't incorporate the term "tweet" into its site until three years after the service started.
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