V1 does appear ‘rubbery’ upon examination, there is the FAR part 1 definition, then FAR part 25.107 has a definition, then I came across this from Big Bleu Airlines:
“V1 – The speed at which Maximum Braking is achieved. The speed must not be greater than Vr, is less then Vmbe (max brake energy) and greater than Vmcg (ground minimum control speed). Reverse thrust is not considered in determining runway limit weights or V1. V1 will be called by the PNF (pilot not flying) prior to attaining V1 on the takeoff roll so that the aborted takeoff will be initiated at V1 and not at a point beyond V1” [italics added]
A bit unwieldy, doesn’t really nail it down, it’s between this and that, not greater than so but less than this. Ah right, thankfully there is a quantitative answer in the speed charts.