The Collaborative International Dictionary of English v.0.48:

Acrostic \A*cros"tic\, n. [Gr. ?; ? extreme + ? order, line,
   verse.]
   1. A composition, usually in verse, in which the first or the
      last letters of the lines, or certain other letters, taken
      in order, form a name, word, phrase, or motto.
      [1913 Webster]

   2. A Hebrew poem in which the lines or stanzas begin with the
      letters of the alphabet in regular order (as Psalm cxix.).
      See Abecedarian.
      [1913 Webster]

   Double acrostic, a species of enigma, in which words are to
      be guessed whose initial and final letters form other
      words.
      [1913 Webster] Acrostic

The Collaborative International Dictionary of English v.0.48:

Acrostic \A*cros"tic\, Acrostical \A*cros"tic*al\, n.
   Pertaining to, or characterized by, acrostics.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acrostic
    n 1: a puzzle where you fill a square grid with words reading
         the same down as across [syn: word square, acrostic]
    2: verse in which certain letters such as the first in each line
       form a word or message

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "acrostic":
   abuse of terms, acronym, amphibologism, amphiboly, anagram,
   avyayibhava, back formation, calembour, clipped word, compound,
   conjugate, construction, corruption, dvandva, dvigu,
   endocentric compound, equivocality, equivoque, exocentric compound,
   formation, jeu de mots, logogram, logogriph, malapropism, metagram,
   missaying, palindrome, paronomasia, paronym, play on words, pun,
   punning, spoonerism, tatpurusha, word form, wordplay