any of a variety of skin structures that secrete oily or greasy substances of various functions. In birds, the preen gland, or uropygial gland, located on the back at the base of the tail, supplies oil that is spread upon the feathers during preening. In mammals, sebaceous glands (sebaceous gland) provide a grease that serves as a protectant and lubricant for hair and skin. Scent glands of certain mammals secrete an often oily material of distinctive odour that serves to mark territorial boundaries.

In some plants the fragrance of flowers is due to essential oils secreted in specialized glands called osmophors. See also preen gland.