A notion is an idea or belief about something, especially one that might not be fully formed or proven yet. When you have a notion that your friend is planning a surprise party, you sense something might be happening but you're not completely sure. When someone says “I have a notion to learn the piano,” they mean the idea has started forming in their mind.

The word often suggests something lighter or less certain than a firm belief or established fact. A scientist might test a notion to see if it holds up to evidence. Your teacher might ask “Where did you get that notion?” if you believe something that isn't quite right. Sometimes people talk about preconceived notions, which are ideas we form about something before we actually experience it ourselves. These notions can be helpful or misleading, which is why it's worth examining them.

A notion can also be a sudden desire or impulse to do something. If your dad wakes up with a notion to make pancakes, he's feeling a spontaneous urge to cook breakfast. Unlike a concrete plan, a notion might fade quickly or might grow into something bigger. The word captures that interesting moment when a thought first appears in your mind, before you've decided what to do with it.