Learn in the textbookThe 7 Elements of Information Security
DefinitionNon-repudiation (否認防止) is the property that a person who performed an operation or sent something cannot later deny it by claiming they did not.
ExplanationThe standard means is a digital signature: a signature only the sender can create stays on the data, so it serves as proof that this person sent it. Because people other than the recipient can also verify the signature, the sending can be proved to third parties as well.
AnalogyLike a contract stamped with your registered personal seal. The impression stays on the page, so you cannot later insist you never made that promise.
Memory hookThe Japanese 否認 reads as "refuse (否) to acknowledge (認)".