An undertaking is a promise to do something or not to do something.
An undertaking may be given between by a party in court proceedings. For example, in civil proceedings an undertaking may be negotiated between parties as a last resort, before a party seeks an injunction.
An undertaking given to the court is punishable as contempt of court if it is breached.
In the context of criminal proceedings, Unused Material is material within the possession/control of the prosecution that does not form part of its case against the defendant.