In context with theatre and movies a scenario (from Italian, that which is pinned to the scenery) is a description of an event or series of actions and events. In the Commedia dell'arte it was an outline of entrances, exits, and action describing the story of a play that was literally pinned to the back of the scenery.
Surviving scenari from the Renaissance contain little other than character names, brief descriptions of action, and references to specific lazzi with no further explanation. It is believed that a scenario forms the basis of a fully improvisational performance though it is also likely that they were simple reminders of the plot for those members of the cast who were literate.
In the creation of an opera or ballet, a scenario is often developed initially to indicate how the original source, if any, is to be adapted. This sketch can be helpful in "pitching" the idea to a prospective producer, director, or composer.
Scenarios are also used in policy planning, and when trying out strategies against uncertain future developments. Here the key idea is for the scenario to be an overview, a summary, of a projected course of action, events or situations. Scenarios are widely used by organizations of all types to understand different ways that future events might unfold.