In linguistics, the protasis is the "if" clause of an "if-then" sentence. Concerning drama, it is the opening part of a play, usually the first act, in which the major characters are introduced. Both meanings have the sense of something that comes before something else. The "if" clause generally comes first in the sentence, and the characters are introduced before the main action occurs in a play.
A complete conditional sentence contains two clauses: a protasis and an apodosis. The protasis is a subordinate clause, meaning that it cannot stand on its own as a complete sentence. An apodosis, or "then" clause, is the main clause. For example, in the sentence, "If it thunders, then the kitten might hide under the bed," "if it thunders" is the protasis, and "then the kitten might hide under the bed" is the apodosis. The word "then" can be left out of the conditional sentence: "If it thunders, the kitten might hide under the bed."
Several different relationships between the protasis and the apodosis are possible. In the sentence above, the protasis causes the apodosis, as thundering causes the kitten to hide. Other types of conditional sentences contain hypothetical situations that are contrary to fact. For instance, the sentence, "If there were bananas in space, then monkeys could live there," contains two clauses that are not true. Syntactically, the sentence implies that if the first clause were true, then the second would be.
The use of this word in drama dates back to the late Classical period. In traditional Greek and Roman theater, the play is usually introduced by a chorus or single actor who gives the appropriate background to the story. For most modern theater, background information is given by dialogue early in the play. Either type of opening can be described as a protasis.
Hamlet, for example, begins with a scene in which two guards see the ghost of Hamlet's father. During the course of the first act, the ghost tells the story of how Hamlet's uncle murdered him. In this way, the protasis sets up the action for the rest of the play.