What is a one actor play called?

It is a play consisting entirely of one act. The act might consist of a few scenes, but no more than would ordinarily occur in the acts of a longer play.
A "one act play" is literally a play in only one act. Usually, plays have 2 or 3 acts, so one acts are shortened and generally very simple plays. They include all the elements of a regular play, they are just much more concise.