You can read more in
this article which defines V1 as:
V1 means the maximum speed in the takeoff
at which the pilot must take the first action
(e.g., apply brakes, reduce thrust, deploy
speed brakes) to stop the airplane within
the accelerate-stop distance. V1 also means
the minimum speed in the takeoff, following
a failure of the critical engine at VEF, at
which the pilot can continue the takeoff and
achieve the required height above the takeoff
surface within the takeoff distance