ACK as an abbreviation that may refer to:
- Acknowledgement (data networks), a signal to acknowledge the receipt of data
- ACK, a flag used in the Transmission Control Protocol (TCP) to acknowledge receipt of a packet
- Acknowledge character, a control character used in ASCII
- Acklington railway station, a rail station in the United Kingdom
- Amar Chitra Katha, an Indian comic book series
- Amsterdam Compiler Kit, a fast, lightweight and retargetable compiler suite
- Anglican Church of Kenya
- Nantucket Memorial Airport, a Massachusetts airport
- ACK Air, a term commonly used to refer to Nantucket Airlines
Ack as a word may also refer to:
- "Ack ack", from signalese for AA, referring to a term for anti-aircraft artillery fire
- Armstrong Whitworth F.K.8, an early aircraft known as the "Big Ack"
- ack, a command-line utility similar to grep that uses the more extensive Perl regular expressions
- short for "Acknowledgement" (sometimes in the sense of acceptance) in online chat slang