So called because their successive verses, or successive parallel series, begin with the successive letters of the alphabet.
Psalm 18, "Blessed are the undefiled," comprises 22 stanzas of 8 verses each, beginning with the same letter of the Hebrew alphabet in order.
Another example of alphabetic arrangement in Hebrew poetry is found in the Lamentations of Jeremias in the Office of Tenebrae during Holy Week.
This feature is not discernible in the Vulgate or our English version save that the Hebrew letter name precedes each verse in the work of the prophet.
New Catholic Dictionary