Noun
1. A bilobate sebaceous gland possessed by the majority of birds. It is located dorsally at the base of the tail (between the fourth caudal vertebrae and the pygostyle) and is greatly variable in both shape and size. In some species, the opening of the gland has a small tuft of feathers to provide a wick for the preen oil (see below). It is a holocrine gland enclosed in a connective tissue capsule made up of glandular acini that deposit their oil secretion into a common collector tube ending in a variable number of pores (openings), most usually two. Each lobe has a central cavity that collects the secretion from tubules arranged radially around the cavity. The gland secretion is conveyed to the surface via ducts that, in most species, open at the top of a papilla (nipple-like structure). (wikipedia.org)
2. A compound alveolar gland of birds located on the dorsum of the tail or pygostyle; the secretion of this gland (fatty acids and wax) exits from a papilla on the dorsal surface at the base of the tail feathers; the bird applies the substance to its feathers by means of the bill when preening. The uropygial gland is lacking in some species but its waterproofing ability is essential to water birds. (biology-online.org)
3. Oil-secreting gland situated at the base of the tail in most birds. (wordnetweb.princeton.edu)
Syn: glandula uropygius,(informal) preen gland, oil gland