Learn in the textbookThe Foundations of Electronic Payment and Data Exchange
DefinitionElectronic money (電子マネー) is a means of payment that records monetary value as electronic data on an IC card or smartphone and pays with that data.
ExplanationIt is one form of cashless payment, and the typical kind is prepaid: like a transit IC card, you pay from a balance you topped up beforehand. You just hold it over a reader, so small payments are over in a moment.
AnalogyPrepaid electronic money is like an electronic purse you fill in advance: each purchase empties it a little, and when it runs dry you fill it up again.
Memory hookThe Japanese word for topping up, チャージ, comes from "charge", as in charging a battery: with the prepaid type, you fill it first, like a phone battery, and then use it.