Taken from FAR PART 25 section 2 - abbreviations and symbols
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
Taken from FAR PART 25 section 107 - takeoff speeds
(a) V1 must be established in relation to VEF as follows:
(1) VEF is the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant, but may noy be less than VmcG determined under Sec. 25.149(e)
(2) V1, in terms of calibrated airspeed, is the takeoff decision speed selected by the applicant, however, V1 may not be less then VEF plus the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's application of the first retarding means during accelerate stop test.
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So for those of you who still want to continue before V1...
