Comic relief (also called episode and interlude) - a humorous scene, incident, or remark occurring in the midst of a serious or tragic literary selection and deliberately designed to relieve emotional intensity and simultaneously to heighten, increase, and highlight the seriousness or tragedy of the action. Apart from being a simple diversion, though, comic relief normally plays some part in advancing the action of drama.
The phrase comes from two words: the first, comic, has the same etymology as that of comedy which is discussed above; relief may be traced from Middle English, back to Middle French, and originally to the Old French relever, meaning “to relieve.”
Since the Sixteenth Century, tragedians have almost universally used comic relief, as in Shakespeare’s drunken porter in Macbeth:
Here’s a knocking indeed! If a man were porter of hell gate, he should have old turning the key. Knock, knock, knock! Who’s there, i’ the name of Belzebub? Here’s a farmer that hanged himself on the expectation of plenty. Come in time! Have napkins enow about you; here you’ll sweat for’t. Knock, knock! Who’s there, in the other devil’s name? Faith, here’s an equivocator, that could swear in both the scales against either scale; who committed treason enough for God’s sake, yet could not equivocate to heaven. O, come in, equivocator! Knock, knock, knock! Who’s there? Faith, here’s an English tailor come hither for stealing out of a French hose. Come in, tailor. Here you may roast your goose. Knock, knock! Never at quiet! What are you? But this place is too cold for hell. I’ll devil-porter it no further. I had thought to have let in some of all professions that go the primrose way to the everlasting bonfire. Anon, anon! I pray you remember the porter.
Act II, scene iii : lines 1 – 19