What does V2 refer to during takeoff?

V2, known as the takeoff safety speed, is critical during the takeoff phase of flight. It is the minimum speed that the aircraft must reach before it can safely lift off the ground and is designed to ensure that the aircraft can continue to climb safely in the event of an engine failure. Achieving V2 ensures that the aircraft has enough performance to clear any obstacles in the takeoff path and maintain a climb without stalling.

This speed is usually calculated based on the aircraft's weight, configuration, and environmental conditions, thereby ensuring safe operational limits are maintained during this critical phase of flight. It's essential for pilots to be aware of V2 because it directly impacts safety and performance after takeoff, especially under adverse conditions or when dealing with emergency scenarios.

In contrast, other terms mentioned in the choices, like decision speed for landing or ground speed at takeoff, refer to different phases of flight or specific operational parameters unrelated to the safety margin during takeoff.