Definition - The clause that expresses the condition in a conditional sentence.

Note: The clause that expresses the consequence is called the apodosis.

Example -

In the sentence If you loved me back, I would love you:
(1) the protasis is If you loved me back and
(2) the apodosis is I would love you.

Etymology -
The word means "condition" in Greek. It derives from the Greek pro, before + teino, stretch.

Oxford English Dictionary -
Its first citation is from 1588:
"Only the protasis or first part of our similitude is attributed but to Cato, for want of a like similitude garnished with like authoritie."
( W. Kempe Educ. Children sig. G.4 v, )

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