The Collaborative International Dictionary of English v.0.48:

Muslin \Mus"lin\, n. [F. mousseline; cf. It. mussolino, mussolo,
   Sp. muselina; all from Mussoul a city of Mesopotamia, Ar.
   Mausil, Syr. Mauzol, Muzol, Mosul, where it was first
   manufactured. Cf. Mull a kind of cloth.]
   A thin cotton, white, dyed, or printed. The name is also
   applied to coarser and heavier cotton goods; as, shirting and
   sheeting muslins. In sheeting, muslin is not as finely woven
   as percale.
   [1913 Webster +PJC]

   Muslin cambric. See Cambric.

   Muslin delaine, a light woolen fabric for women's dresses.
      See Delaine. [Written also mousseline de laine.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

muslin
    n 1: plain-woven cotton fabric