Source : Webster's Revised Unabridged Dictionary (1913)

Marble \Mar"ble\, v. t. [imp. & p. p. {Marbled}; p. pr. & vb. n.
   {Marbling}.] [Cf. F. marbrer. See {Marble}, n.]
   To stain or vein like marble; to variegate in color; as, to
   marble the edges of a book, or the surface of paper.

Marbling \Mar"bling\, n.
   1. The art or practice of variegating in color, in imitation
      of marble.

   2. An intermixture of fat and lean in meat, giving it a
      marbled appearance.

   3. pl. (Zo["o]l.) Distinct markings resembling the
      variegations of marble, as on birds and insects.

Source : WordNet®

marbling
     n : the intermixture of fat and lean in a cut of meat