They not be quite what you're picturing; nowadays, "aircraft carrier" summons up an image of a floating airfield nearly the size of a Georgia county.*
But you have to start somewhere; the British started before the 20th Century was two decades old, with the Thorneycroft Seaplane Lighter: a ship. That carried aircraft. An aircraft, in fact: singular. And it was on its own for the taking off and landing.
It might sound a little mad but the concept worked so well for the Royal Navy that they built 46 of them. There was one "survivor" known, but it was stuck in the mud and corroded away.
And then a maritime journalist spotted one in the Thames -- stuck, rusting, but remarkably intact!
It is presently being restored and will be displayed at the Fleet Air Arm Musem: an aircraft carrier not much larger than a modest modern houseboat; but an aircraft carrier all the same.
* Originally required to have no border farther than a half-day's horseback ride from the county seat. Not a bad idea, especially in light of modern-day fuel prices.