Plain English Meaning
The spot on the runway where you line up straight with the centerline, ready to start your takeoff run.
Full Definition
The location on the runway where the airplane is aligned with the runway centerline and stopped or rolling at the point from which the takeoff roll will begin, after entering from a taxiway and being cleared (or, at a non-towered field, having confirmed it is safe) to depart.
Context Anchor
Seen during preflight and before-takeoff checks, especially when confirming items such as flaps, trim, seats, or controls are set correctly.