(1) the act of signing a person's name to a check or other financial instrument, usually on the back, to indicate the legal transfer of ownership of the instrument, especially in return for money or credit indicated on the face of the instrument. (2) to sign a note as a third party guaranteeing payment in the even of default by the principal borrower. (3) to sign a contract indicating approval of its contents or terms. (4) to acknowledge with a signature the receipt of payment. (5) to give support or sanction to something.