What is a Tweet?
A tweet is a post on Twitter, the social media website. Twitter's logo is a blue bird, and "tweeting" means something like the "chirping" of birds.
Several social media have tried to emulate the brand power of the verb "tweet."
Note that although Tumblr and Pinterest use the terms "reblog" and "repin," akin to Twitter's "retweet," it's not necessarily the case one was inspired by the other.
After being rebranded to "X," tweets started being (officially, at least) called "posts." Everyone else still calls them tweets.