To be overjoyed means to feel extremely happy, so happy that joy seems to overflow inside you. It's that wonderful, almost overwhelming happiness that makes you want to jump up and down or shout with excitement.

You might feel overjoyed when you find out your family is adopting the puppy you've been hoping for, or when you learn your best friend is moving back to town after being away for a year. A musician might be overjoyed after giving a performance that goes even better than they imagined, or a scientist might feel overjoyed after years of work finally lead to an important discovery.

The “over” part of the word tells you something important: this joy is so strong it goes over the normal level of happiness. It's the kind of feeling that lights up your whole face and makes it hard to sit still. When you're overjoyed, people can usually tell just by looking at you, because that kind of happiness is hard to hide.