To imagine means to form pictures, ideas, or scenarios in your mind that aren't actually in front of you right now. When you imagine something, you're creating a kind of mental movie or thought experiment. You might imagine what it would be like to fly, picture your friend's reaction to a surprise party, or think through how to rearrange your bedroom before moving any furniture.

Imagination is one of the most powerful tools humans have. Engineers imagine bridges before they build them. Authors imagine entire worlds and characters before writing them down. Scientists imagine possible explanations for mysteries in nature, then test whether their ideas are correct. Every invention, from the wheel to the smartphone, started as something someone imagined.

You use your imagination constantly: when you read a book and picture the characters, when you daydream about summer vacation, when you plan your strategy in a game, or when you try to understand how someone else might be feeling. Young children often have especially vivid imaginations, creating elaborate pretend games or imaginary friends.

The phrase “imagine that!” expresses surprise, as in “She read 50 books this summer. Imagine that!” Sometimes people say “I can't imagine” to mean something is extremely difficult to picture or believe: “I can't imagine climbing Mount Everest.”