The Collaborative International Dictionary of English v.0.48:

Vellum \Vel"lum\, n. [OE. velim, F. v['e]lin, fr. L. vitulinus
   of a calf, fr. vitulus a calf. See Veal.]
   A fine kind of parchment, usually made from calfskin, and
   rendered clear and white, -- used as for writing upon, and
   for binding books.
   [1913 Webster]

   Vellum cloth, a fine kind of cotton fabric, made very
      transparent, and used as a tracing cloth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vellum
    n 1: a heavy creamy-colored paper resembling parchment
    2: fine parchment prepared from the skin of a young animal e.g.
       a calf or lamb