Iambic pentameter is a line of verse with five iambic feet, each consisting of an unstressed syllable followed by a stressed one.
Iambic pentameter is a metre of five iambs — ten syllables alternating unstressed-STRESSED, unstressed-STRESSED — that became the dominant line of English verse from Chaucer onward. Geoffrey Chaucer used it in 'The Canterbury Tales' (c. 1387); Shakespeare made it the default voice of his plays, both in unrhymed blank verse and in the rhymed couplets that close his sonnets. Milton used it for 'Paradise Lost' (1667), Wordsworth for 'The Prelude', Robert Browning for his dramatic monologues. The metre's appeal is that it sits naturally close to spoken English without becoming prose — a practised ear hears the underlying drum but never feels battered by it. Even contemporary poets who write 'free verse' often slip into iambic pentameter at moments of high feeling, because the rhythm is so deeply learned that it has become English's default solemn speech.