Plain English Meaning
A small box of motion sensors that feels how the aircraft is turning, tilting, and accelerating, then sends that information to the system that draws the attitude and heading on your display.
Full Definition
A self-contained electronic unit within an Attitude and Heading Reference System (AHRS) that houses the motion-sensing components — typically three-axis solid-state gyroscopes, accelerometers, and magnetometers — used to detect the aircraft's rotation, acceleration, and magnetic heading. The ISA feeds raw sensor data to the AHRS processor, which calculates aircraft attitude and heading for display on the .