The Collaborative International Dictionary of English v.0.48:

Monogram \Mon"o*gram\, n. [L. monogramma; Gr. mo`nos single +
   gra`mma letter, fr. gra`fein to write: cf. F. monogramme. See
   Graphic.]
   1. A character or cipher composed of two or more letters
      interwoven or combined so as to represent a name, or a
      part of it (usually the initials). Monograms are often
      used on seals, ornamental pins, rings, buttons, and by
      painters, engravers, etc., to distinguish their works.
      [1913 Webster] Monogram.
      [1913 Webster]

   Note: The monogram above, combining the letters of the name
         Karolvs, was used by Charlemagne.
         [1913 Webster]

   2. A picture in lines; a sketch. [R.]
      [1913 Webster]

   3. An arbitrary sign for a word. [R.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

monogram \mon"o*gram\, v. t.
   To inscribe or ornament with a monogram.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

XP \XP\ [Belongs here in appearance only.]
   The first two letters of the Greek word XRISTOS, Christ; --
   an abbreviation used with the letters separate or, oftener,
   in a monogram, often inclosed in a circle, as a symbol or
   emblem of Christ. It use as an emblem was introduced by
   Constantine the Great, whence it is known as the

   Constantinian symbol, or

   monogram. See Labarum.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

monogram
    n 1: a graphic symbol consisting of 2 or more letters combined
         (usually your initials); printed on stationery or
         embroidered on clothing

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "monogram":
   ID card, John Hancock, X, alphabetic character, autograph, badge,
   calligram, calling card, card, character, christcross, cipher,
   countermark, countersign, countersignature, counterstamp,
   credentials, cross, device, dog tag, endorsement, graph, grapheme,
   hand, identification, identification badge, identification tag,
   initials, letter, letter of introduction, lexigraphic character,
   mark, mark of signature, phonetic character, phonetic symbol,
   pictographic character, press card, seal, serial number, sigil,
   sign, sign manual, signature, signet, subscription, syllabic,
   symbol, tessera, visa, vise, visiting card, writing,
   written character



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MONOGRAM. A character or cipher composed of one or more letters interwoven,
being an abbreviation of a name.
     2. A signature made by a monogram would perhaps be binding, provided it
could be proved to have been made and intended as a signature. 1 Denio, R.
471. And there seems to be no reason why such a signature should not be as
binding as one which is altogether illegible. See Initial; Mark; Signature.