1 over square root of 2 or 1/sqrt(2) = 0.70710678118655.
This is the effective level of voltage or DC equivalent level of voltage for AC, the alternating voltage supply, which generates the same amount of power as DC.
It is calculated using the ROOT MEAN SQUARED method using calculus over a period of 60 hertz or 1/60 of a second.
Normally it is said that 70.7% of the AC peak voltage is the RMS value, ie, the effective value.
A 200 volts peak AC voltage will have 141.4 volts effective voltage which is 70.7% of 200.