Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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}.

   {Double acrostic}, a species of enigma

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

Source : WordNet®

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