Plain English Meaning
A system that knows where the aircraft is by tracking every movement it has made since takeoff, using sensors that feel motion. It does not need any signal from the outside world to do this.
Full Definition
A self-contained navigation system that continuously calculates an aircraft's position, velocity, and attitude by measuring its own accelerations and rotations from a known starting point, without relying on external signals such as ground stations or satellites.
Context Anchor
Seen in advanced aircraft navigation discussions, especially when describing systems that can continue providing navigation information without relying on ground stations or satellite signals.