acrostic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A poem or series of lines in which certain letters, usually the first in each line, form a name, motto, or message when read in sequence.

from The Century Dictionary.

• Crossed; folded across; crossing.
• noun A composition in verse, in which the first, or the first and last, or certain other letters of the lines, taken in order, form a name, title, motto, the order of the alphabet, etc.
• noun A Hebrew poem in which the initial letters of the lines or stanzas were made to run over the letters of the alphabet in their order. Twelve of the Psalms are of this character, of which Psalm exix. is the best example.
• Pertaining to, of the nature of, or containing an acrostic: as, acrostic verses.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun A Hebrew poem in which the lines or stanzas begin with the letters of the alphabet in regular order (as Psalm cxix.). See Abecedarian.
• noun a species of enigma, in which words are to be guessed whose initial and final letters form other words.
• noun Pertaining to, or characterized by, acrostics.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A poem or other text in which certain letters, often the first in each line, spell out a name or message.
• noun A particular kind of word puzzle: its solutions form an anagram of a quotation, and their initials often form its author.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun verse in which certain letters such as the first in each line form a word or message
• noun a puzzle where you fill a square grid with words reading the same down as across