noun

• The act of acknowledging
• The act of recognizing in a particular character or relationship; recognition of existence, authority, truth, or genuineness.
• A reward or other expression or token of gratitude.
• An expression of gratitude for a benefit or an obligation.
• A message from the addressee informing the originator that the originator's communication has been received and understood.
• (telecommunications, computing, networking) A response (ACK) sent by a receiver to indicate successful receipt of a transmission (See Wikipedia article on transmission).
• A recognition as genuine or valid; an avowing or admission in legal form.
• (law) The act of a person admitting a child as their own.
• (law) A formal statement or document recognizing the fulfillment or execution of a legal requirement or procedure.