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