Yay is an exclamation of joy, excitement, or triumph. When you shout yay! after your team scores the winning goal, or when your teacher announces a surprise field trip, you're expressing pure happiness in its simplest form.

The word captures that burst of positive feeling when something good happens. A younger sibling might jump up and down yelling “Yay! Yay! Yay!” when they learn they're going to the zoo. Your friend might text you “yay!” when you tell them you can come to their birthday party.

Yay is completely informal. You wouldn't write it in a serious essay or use it in a formal speech. But in everyday conversation, texts, and casual writing, it's a cheerful way to show enthusiasm. Some people draw it out for emphasis: “Yaaaay!” The longer you stretch it, the more excited you sound.

Similar words include hooray, hurrah, and woohoo, but yay is probably the simplest and most commonly used today. It's the sound of uncomplicated happiness, the verbal equivalent of a smile.