Theoretical means based on ideas and reasoning rather than on practical experience or real-world testing. When scientists develop a theoretical model of how black holes work, they're using mathematics and logic to predict what happens, even though they can't visit a black hole to check. A theoretical physicist spends time working out equations and concepts on paper or computer, exploring how the universe might work.

The word often contrasts with “practical” or “experimental.” You might understand multiplication theoretically, knowing that 7 times 8 equals 56, but applying it practically when you're figuring out how many cookies you need for seven friends getting eight cookies each. A theoretical possibility is something that could happen according to logic, even if it's never actually occurred. It's theoretically possible to flip a coin and get heads 100 times in a row, though the chances are vanishingly small.

Sometimes people use theoretical to mean something exists only as an idea, not in reality. A theoretical solution to a problem might work perfectly on paper but fail when you try it in the messy real world. When someone dismisses an argument as “purely theoretical,” they mean it sounds good as an idea but won't actually work in practice.