Non-repudiation

Assurance that the sender of information 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 information.
SOURCE: CNSSI-4009; SP 800-60

Protection against an individual falsely denying having performed a particular action. Provides the capability to determine whether a given individual took a particular action such as creating information, sending a message, approving information, and receiving a message.
SOURCE: SP 800-53; SP 800-18

Is the security service by which the entities involved in a communication cannot deny having participated. Specifically, the sending entity cannot deny having sent a message (non-repudiation with proof of origin), and the receiving entity cannot deny having received a message (non-repudiation with proof of delivery).
SOURCE: FIPS 191

A service that is used to provide assurance of the integrity and origin of data in such a way that the integrity and origin can be verified and validated by a third party as having originated from a specific entity in possession of the private key (i.e., the signatory).
SOURCE: FIPS 186