Plain English Meaning
The colored lights on the wingtips and tail that show others which way the aircraft is pointing and moving, especially at night.
Full Definition
The red, green, and white navigation lights mounted on an aircraft to indicate its orientation and direction of travel to other pilots and ground personnel. A red light is on the left wingtip, a green light is on the right wingtip, and a white light is at the tail. Required to be on from sunset to sunrise under FAR 91.209.
Context Anchor
You will see this term in checklists and procedures for night operations, engine starting, , and light checks.