n The act of owning or recognizing in a particular character or relationship; recognition as regards the existence, authority, truth, or genuineness.
n A message from the addressee informing the originator that the originator's communication has been received and understood.
n (telecommunications|computing|networking) A response sent by a receiver to indicate successful receipt of a transmission.
n (context|legal) A formal statement or document recognizing the fulfilment or execution of a legal requirement or procedure.