The peak voltage of an alternating current is the maximum or minimum voltage of a sinusoidal wave. Rather than using instantaneous power, it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value.
More info: http://en.wikipedia.org/wiki/Alternating_current#Power_and_root_mean_square