The acronym DEFENSE AERIAL is composed of three parts: SAC (3 letters), ACK (3 letters), and the word 'DEFENSE'. SAC stands for Strategic Air Command, which was a major command of the United States Air Force from 1946 to 1992. ACK stands for Acknowledgement, which is a signal sent by a receiver to indicate that it has received and understood a message. Finally, the word 'DEFENSE' refers to the act of protecting or guarding against attack, danger, or injury. Together, these three components form the acronym DEFENSE AERIAL, which is used to refer to the DEFENSE of airspace.