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