1. The capability, in security systems, that guarantees that a message or data can be proven to have originated from a specific person. 2. Assurance the sender of data is provided with proof of delivery and the recipient is provided with proof of the sender's identity, so neither can later deny having processed the data. 3. The property of a receiver being able to prove that the sender of some data did in fact send the data even though the sender might later deny ever having sent it.