1. (grammar) the clause expressing the condition in a conditional sentence, typically beginning with "if"
   "In the sentence 'If it rains, we'll stay home,' 'If it rains' is the protasis"
    
2. (literature) the introductory part of a play or poem, setting forth the subject
   "The protasis of the epic introduced the main characters and their conflicts"

Derived forms: protases

Encyclopedia: Protasis