The average of an AC waveform's instantaneous values taken over a half cycle. Ordinary testers respond to the average value.
For sinusoidal wave :
Average value = Maximum value×2/π = Maximum value×0.637
When the true RMS value is 100V ;
Average value= Maximum value×2/π =141×0.637 = 90(V)
The reading of ordinary testers is calibrated in terms of the effective value of a sinusoidal wave even though they are responding to the average value. They are called average-responding-RMS-calibrated type of testers. As opposed to these, true-RMS type testers respond and show the true RMS value.