Theoretically means according to theory or in principle, rather than based on actual experience or proven facts. When you say something is theoretically possible, you mean it could work according to the rules or logic you understand, but you haven't actually tested it or seen it happen yet.

A student might say, “Theoretically, I could finish all my homework in one hour if I focused perfectly without any distractions.” The math checks out, the plan makes sense, but in reality, interruptions happen, concentration wavers, and it usually takes longer. Scientists use the word when describing ideas they've worked out on paper but haven't yet tested in experiments. An engineer might say a bridge design will theoretically support a certain weight based on calculations, but they'll still want to test it thoroughly before anyone drives across.

The word often appears when someone is being careful to distinguish between what should happen according to their understanding and what actually happens in the real world. When your friend explains a complicated plan and you respond, “Theoretically, yes, but...” you're pointing out that while the logic seems sound, practical problems might get in the way. It's the difference between an idea on paper and an idea tested by reality.