The rms (root mean square) value is the effective value of AC that produces the same average heating effect as a DC voltage of that value. For a sinusoidal wave $V(t)=V_0\sin (\omega t)$: $V_{rms}=\sqrt{\frac{1}{T}{\int }_0^{T}V(t{)}^{2}dt}$ $V_{rms}=\frac{V_0}{\sqrt{2}}\approx 0.707V_0$. Standard household voltage (e.g., $230\text{ V}$) is given as the rms value.