Plain English Meaning
The instant you let the brakes go and the airplane starts rolling down the runway for takeoff.
Full Definition
The moment during the takeoff procedure when the pilot lifts their feet off the wheel brakes, allowing the airplane to begin its takeoff roll. It marks the official start of the takeoff and is the reference point from which takeoff distance, time, and performance are measured.
Context Anchor
You will see this term in takeoff procedures and performance descriptions, especially when measuring distance from the start of the takeoff run.