1. a line on a contract or similar document for a party's signature.
2. a line at which a sheet of paper is perforated or a piece of it is to be detached.
3. signature on the dotted line, full agreement to terms and conditions.
4. sign on the dotted line, to agree fully to terms or conditions.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.