Also called an adventure.
The scenario is the basic idea, plot and characters used by the gamemaster in a roleplaying game.
The scenario can exist either as a loose idea in the mind of the gamemaster or as a written down, layoutet production produced by someone or even published. Most roleplaying games also have a number of published scenarios.
A scenario can be in a number of genres, settings, and moods. Can be written (or thougt!) for a particular world or rulesystem or for no particular one.