The definition of a gland is an organ or group of cells that releases substances or waste from the body. An example of a gland is the thyroid. any organ or specialized group of cells that produces secretions, as insulin or bile, or excretions, as urine: some glands, as the liver and kidneys, have ducts that empty into an organ: the ductless (or endocrine) glands, as the thyroid and adrenals, secrete hormones.