ZnO discs, acting as a varistor, are the active material in most surge arresters. The name "varistor" is a portmanteau of "varying resistor". The main parameter affecting varistor life expectancy is its energy (Joule) rating. This type is commonly known as the metal-oxide varistor (MOV). The most common modern type of varistor is the metal-oxide varistor (MOV). Follow-through current resulting from a strike may melt, burn, or even vaporize the varistor. This is an abbreviated combination of the words "transconductance" or "transfer", and "varistor". One of the most widely used of these is the varistor.