- any of various glands, as the thyroid, adrenal, and pituitary glands, that secrete certain substances or hormones directly into the blood or lymph; ductless gland.
- any of the glands that secrete hormones directly into the bloodstream, including the pituitary, pineal, thyroid, parathyroid, adrenal, testes, ovaries, and the pancreatic islets of Langerhans
