Protasis definition

Latin protasis, from Greek προτασις, from προτεινειν ‘put forward, tender’, from προ− + τεινειν ‘stretch’.

the first part of a play, in which the setting and characters are introduced
- 1922: It doubles itself in the middle of his life, reflects itself in another, repeats itself, protasis, epitasis, catastasis, catastrophe. — James Joyce, Ulysses
the antecedent in a conditional sentence

example: In, "I will be coming if this weather holds up", "this weather holds up" is the protasis