A main character in Lewis Carroll's "Alice in Wonderland," the Mad Hatter is, as the name implies, a seemingly insane man, ostensibly a hat maker, who is obsessed with tea time and tea parties. Displaying poor manners and a desire to confuse and challenge tea party guests with riddles, the Mad Hatter speaks to Alice at a highly disordered tea party.
Deriving his name from the slang term "mad as a hatter," in which "mad" means "insane," the Mad Hatter himself is mentally disturbed for different reasons than the hatters referenced in the slang term; harsh chemicals were once used in hatmaking, which made haberdashers and other hatmakers subject to mental health deficiencies. The Mad Hatter in Carroll's tale has lost his mind due to a series of legal conflicts with Wonderland's queen, leaving him shaken and unsure of the passage of time.
Though this character is widely known as the Mad Hatter, he is never referred to directly as such in the book; he is referred to as "Hatter." However, it is well known both to the observant reader who tracks the Hatter's erratic behavior and to the characters in the book; the Cheshire Cat warns Alice of the Mad Hatter's mental instability when they meet in the forest.