Pronunciation: (prot'u-sis),[key] —n., —pl.-sesPronunciation: (-sēz").[key] 1. the clause expressing the condition in a conditional sentence, in English usually beginning with if. Cf. apodosis. 2. the first part of an ancient drama, in which the characters are introduced and the subject is proposed. Cf. catastasis, catastrophe (def. 4), epitasis. 3. (in Aristotelian logic) a proposition, esp. one used as a premise in a syllogism.