An abbreviation for acknowledged. ASCII character ACK 0x06 (^F), used to acknowledge reception of a block of data in simple serial protocols, may also be sent in reply to an ENQ 0x05 (^E).
Also used as the name for a pulse in the electrical protocol of a chip or for one of its pins.
Sometimes used in online and/or spoken language between nerds. Not to be confused with ack!, an exclamation of dismay.