In law, non-repudiation is a situation where a statement's author cannot successfully dispute its authorship or the validity of an associated contract.[1] The term is often seen in a legal setting when the authenticity of a signature is being challenged. In such an instance, the authenticity is being "repudiated".[2]
For example, Mallory buys a cell phone for $100, writes a paper cheque as payment, and signs the cheque with a pen. Later, she finds that she can't afford it, and claims that the cheque is a forgery. The signature guarantees that only Mallory could have signed the cheque, and so Mallory's bank must pay the cheque. This is non-repudiation; Mallory cannot repudiate the cheque. In practice, pen-and-paper signatures aren't hard to forge, but digital signatures can be very hard to break.