The Collaborative International Dictionary of English v.0.48:

Uropygial \U`ro*pyg"i*al\, a. [See Uropygium.] (Anat.)
   Of or pertaining to the uropygium, or prominence at the base
   of the tail feathers, in birds.
   [1913 Webster]

   Uropygial gland, a peculiar sebaceous gland at the base of
      the tail feathers in most birds. It secretes an oily fluid
      which is spread over the feathers by preening.
      [1913 Webster]

WordNet (r) 3.0 (2006):

uropygial gland
    n 1: oil-secreting gland situated at the base of the tail in
         most birds [syn: uropygial gland, preen gland]