what a communication that is about something is about

a list of acknowledgements of those who contributed to the creation of a film (usually run at the end of the film)

an acknowledgment (usually tangible) that payment has been made

an acknowledgment or expression of goodwill at parting

(usually plural) an acknowledgment or expression of good will (especially on meeting)

an acknowledgment that can be used to say hello or goodbye (aloha is Hawaiian and ciao is Italian)