Catalectic

A catalectic line is a metrically incomplete line of verse, lacking a syllable at the end or ending with an incomplete foot. One form of catalexis is headlessness, where the unstressed syllable is dropped from the beginning of the line.

Making a meter catalectic can drastically change the feeling of the poem, and catalexis is often used to achieve a certain effect. Compare this selection from Book III of Henry Wadsworth Longfellow's "Song of Hiawatha" with that from W. H. Auden's "Lay Your Sleeping Head, My Love". The first is in trochaic tetrameter (a trochaic dimeter in ancient terms, as a Greek trochaic metron is a doubled 'trochee'), and the second in trochaic tetrameter catalectic (or headless iambic tetrameter).