What is the difference between malapropism and peripetia?
The term Malapropism refers to a linguistic incongruity having derived its name from the character of Mrs. Malaprop in Sheridan's comedy, The Rivals. French mal a propos means "ill to the purpose", i.e. inappropriate. Mrs. Malaprop, an elderly widow in Sheridan's play, was in the habit of using words inappropriately, e.g. "a nice derangement of epitaphs" for "a nice arrangement of epithets". Such inappropriate use of words/phrases is called Malapropism.
The term Peripetia owes its origin to Aristotle's Poetics. It is a Greek word which means "reversal". Aristotle refers to this term to suggest a reversal of fortunes for the protagonist in tragedy. Peripety occurs in Sophocles's tragedy when the messenger from Corinth comes to inform Oedipus that king Polybus and queen Merope are dead, and he further tells the king of Thebes that Polybus and Merope were not his parents. His father was Laius and mother was Jocasta.