A kind of metrical foot. An iamb (the adjective is "iambic") is an unstressed syllable followed by a stressed one.
They álso sérve who ónly stánd and wáit.
Because each foot consists of two syllables, the iambic is known as a duple meter. The other most common duple meter is trochaic. Note that you can distinguish iambic from trochaic meter only by examining the first and last feet in a verse, since they both alternate stressed and unstressed syllables.

From the Guide to Literary Terms by Jack Lynch.
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