[OZ4]

An ac voltage is dynamic it's constantly changing.  Because it's constantly changing it's voltage is not a single value. The peak value is the highest value seen. If I remember correctly to find peak value you multiply by the square root of two. 

The voltage reading from a typical outlet is said to be approximately 117vac.  The ac value assigned is from the power that's equivalent to 117VDC when either are measured into same resistor or resistive load.