mailbox rule

noun | mail·box rule

1. : a rule treating the sending of something as constituting a filing or as a basis for assuming receipt; specifically : a rule in contract law: a notice of acceptance of an offer sent to the offeror by reasonable means or as agreed by the parties is effective and is not affected by a notice of revocation of the offer received later : a rule in contract law: a notice of acceptance of an offer sent to the offeror by reasonable means or as agreed by the parties is effective and is not affected by a notice of revocation of the offer received later