the clause expressing the condition in a conditional sentence, in English usually beginning with if.Compare apodosis.
2.
the first part of an ancient drama, in which the characters are introduced and the subject is proposed. Compare catastasis, catastrophe(def. 4), epitasis.
3.
(in Aristotelian logic) a proposition, esp. one used as a premise in a syllogism.
Origin: 1610–20; < LL: introduction in a drama < Gk prótasis proposition, lit., a stretching forward, equiv. to pro-pro-2+ tásis a stretching (ta-, verbid s. of teínein to stretch + -sis-sis)
Grammar The dependent clause of a conditional sentence, as if it rains in The game will be canceled if it rains.
The first part of an ancient Greek or Roman drama, in which the characters and subject are introduced.
[Late Latin, proposition, first part of a play, from Greek, premise of a syllogism, conditional clause, from proteinein, prota-, to propose : pro-, forward; see pro-2 + teinein, to stretch; see ten- in Indo-European roots.] pro·tat'ic (prŏ-tāt'ĭk, prō-) adj.
[prot-uh-sis] 
ə