Short for acknowledged. When a person says something that doesn't require a response, but the person wants to let them know they recieved the message. Similar to okay, but when using okay just won't do.
Sanford: I had cheerios for breakfasttoday.
Stephen: I don't care
Sanford: Ack
In the TCP protocol, the receiving computer sends an ACK (acknowledgment) to the sending computer. This is basically how a coonection is established.
Sender sends 3 packets, Receiver receives 3 packets and sends "ACK 4" (meaning it received what was sent, and it's ok to send the nextnumber or packet)