Plain English Meaning
The last set of checks the pilot does after taxiing and before lining up to take off. The airplane is stopped (usually near the runway), and the pilot runs through the checklist to make sure the engine, instruments, controls, and switches are all set correctly for departure.
Full Definition
The phase of flight operations between completing taxi and applying takeoff power, during which the pilot performs the before-takeoff check — a structured sequence of engine run-up, system checks, control checks, and final cockpit setup — to confirm the airplane is airworthy and properly configured for departure.