What is the crest factor in a power supply?

The crest factor is the ratio of the peak value to the effective value of a parameter. A sinusoidal (sine-wave-shaped) alternating current voltage with an effective value of 230 V and a peak value of 325 V has a crest factor of 1.41. 

The crest factor is of practical importance for AC-UPS systems. They specify a maximum crest factor or a maximum peak value for the input current. The crest factor is given in the product datasheets of PULS power supplies.