Something added to complete a thing, make up for a deficiency, or extend or strengthen the whole, That which fills up, completes, or makes an addition to, something already organized, arranged, or set apart; specifically, a part added to, or issued as a continuation of, a book or paper, to make good its deficiencies or correct its errors, Something added, especially to make up for a deficiency, verb_ add as a supplement to what seems insufficient, add to the very end, To provide or make a supplement to something