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 cataletic can drastically change the feeling of the poem, and 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, and the second in trochaic tetrameter catalectic (or headless iambic tetrameter).
- By the shores of Gitche Gumee,
- By the shining Big-Sea-Water,
- Stood the wigwam of Nokomis,
- Daughter of the Moon, Nokomis.
- Dark behind it rose the forest,
- Rose the black and gloomy pine-trees,
- Rose the firs with cones upon them;
- Bright before it beat the water,
- Beat the clear and sunny water,
- Beat the shining Big-Sea-Water.
--H. W. Longfellow
--W. H. Auden
A line missing two syllables is called brachycatalectic.
Ancient and Musical Use 
Very few ancient Greek poems survive with authentic musical notation. Four of these are by Mesomedes (early 2nd century CE). The Sphinx employs a form of anapestic dimeter and its catalectic form, the paroemiac. Secondary sources of Mesomede's poems To Helios and To Nemesis are in a catalectic meter known as apokrota "sonorous." In each case, in place of the missing short element of the text (i.e., missing syllable) one often finds lengthening signs. In two cases in To Helios, this appears to be a three-note melisma. It is possible ancient use of catalexis indicated some form of melody or continued singing in place of the missing syllables.
In ancient Greek drama, catalectic meters may have been associated with a male aulete or had some other special use. For example, of Menander's surviving plays, almost all are in iambic trimeters. He changed the meter in one long scene in Misanthrope to 15-syllable catalectic iambic tetrameter recited to an aulos accompaniment.
As Greek meter is often used to describe musical phrasing, some famous themes include:
- The opening theme to Mozart's violin sonata in B-flat (Trochaic Dimeter Catalectic)
- The slow movement to Haydn's Surprise Symphony (Spondaic dimeter catalectic)
- The theme of Webers Rondo brillante in E-flat (Anapestic Tetrameter brachycatectic)
- The slow movement of Beethoven's 7th symphony (alternating acatalectic and catalectic dactylic tetrameter)
In all these cases, the catalectic "syllable" represents a held note (in which a player may choose to introduce or withhold a "breath"). It is a place where the metrical melody is subject to harmonic reinforcement.
See also 
- West, M.L. (1992). Ancient Greek Music. Oxford: Oxford. pp. 209, 302–308.
- Comotti, G. (1975). "L'aulo ghingras in una scena menandrea del mosaico di Discuride". Quaderni urbinati di cultura classica xx: 215–23.
- Norberg, D. (1988). "Le "Pange lingue" de Fortunat pour la Croix". La Maison-Dieu 103: 71–79.