The Collaborative International Dictionary of English v.0.48:

Tunic \Tu"nic\ (t[=u]"n[i^]k), n. [L. tunica: cf. F. tunique.]
   [1913 Webster]
   1. (Rom. Antiq.) An under-garment worn by the ancient Romans
      of both sexes. It was made with or without sleeves,
      reached to or below the knees, and was confined at the
      waist by a girdle.
      [1913 Webster]

   2. Any similar garment worn by ancient or Oriental peoples;
      also, a common name for various styles of loose-fitting
      under-garments and over-garments worn in modern times by
      Europeans and others.
      [1913 Webster]

   3. (R. C. Ch.) Same as Tunicle.
      [1913 Webster]

   4. (Anat.) A membrane, or layer of tissue, especially when
      enveloping an organ or part, as the eye.
      [1913 Webster]

   5. (Bot.) A natural covering; an integument; as, the tunic of
      a seed.
      [1913 Webster]

   6. (Zool.) See Mantle, n., 3
      (a) .
          [1913 Webster]

WordNet (r) 3.0 (2006):

tunic
    n 1: an enveloping or covering membrane or layer of body tissue
         [syn: tunic, tunica, adventitia]
    2: any of a variety of loose fitting cloaks extending to the
       hips or knees