Definition
The maximum speed during the takeoff roll at which the pilot must take the first action to reject the takeoff if a problem occurs. Above V1, the takeoff must be continued, because there is no longer enough runway remaining to stop the aircraft safely. V1 is calculated for each takeoff based on aircraft weight, runway length, runway condition, and other performance factors.
Plain English
V1 is the speed during takeoff after which you are committed to flying. Before V1, you can still stop on the runway if something goes wrong. After V1, you must keep going and lift off, because there isn't enough runway left to stop.
Context Anchor
You see V1 in takeoff planning, performance charts, cockpit callouts, and multi-engine takeoff briefings.
Derivation
The 'V' stands for velocity, and the '1' marks it as the first of the defined takeoff speeds (followed by VR for rotation and V2 for initial climb). Knowing V is simply 'velocity' helps the pilot read the whole family of V-speeds without confusion.
Why Pilots Care
It marks the point where continuing the takeoff becomes the safer choice; aborting after V1 can result in running out of runway.
Grounding Statement
Picture the airplane accelerating down the runway: V1 is the point where the plan changes from “stop if needed” to “continue the takeoff.”
Intuition Check
V1 is not the speed where the airplane lifts off. V1 is the decision and action speed for stopping or continuing the takeoff.
Example Sentence 1
The captain briefed that any failure before V1 would result in a rejected takeoff, and any failure after V1 would mean continuing into the air.
Example Sentence 2
During training the instructor emphasized that once the airplane passes V1 the only option is to fly.