Simple Definition:

Electrolysis is a process that uses an electric current to break down a substance (called an electrolyte) into its component parts [parts that make up the substance]. For example, water (an electrolyte) can be split into hydrogen gas and oxygen gas through electrolysis.

Very Simple Definition:

Electrolysis is when electricity is used to split a substance into its parts. For example, splitting water into two gases: hydrogen and oxygen.