Extravagance means spending money or using resources in a way that goes far beyond what's necessary, often wastefully or excessively. When someone buys an extravagance, they're purchasing something luxurious they don't really need: perhaps a gold-plated bicycle when a regular one works just fine, or a massive ice cream sundae when a single scoop would satisfy.

The word carries a sense of going overboard. There's a difference between treating yourself to something nice and true extravagance. Buying yourself a new book is a treat; buying fifty books you'll never read is extravagant. Ordering pizza for dinner is normal; ordering ten pizzas for three people is extravagant.

Extravagance isn't always about money. Someone might be extravagant with compliments, praising everything so enthusiastically that the words lose meaning. A stage production might feature extravagant costumes covered in sequins, feathers, and jewels, designed to dazzle the audience.

The adjective form is extravagant. A king might throw an extravagant party with fountains of chocolate and fireworks, while his practical advisor worries about the extravagance of the expense. Sometimes extravagance creates memorable experiences, but it can also lead to waste or financial trouble when people spend beyond their means.