1. To give one's support or approval to;
2. Said of documents or checks: to sign in order to complete or to render valid.
3. To write on the back of; especially; to sign one's name as payee on the back of (a check) in order to obtain the cash or credit represented on the face; to inscribe (one's signature) on a check, bill, or note; to inscribe (as an official document) with a title or memorandum; to make over to another (the value represented in a check, bill, or note) by inscribing one's name on the document; to acknowledge receipt of (a sum specified) by one's signature on a document
4. To approve openly ; especially; to express support or approval of publicly and definitely; to recommend (as a product or service) usually for financial compensation