Probably means something is likely to happen or be true, but you're not completely certain. When you say something will probably happen, you're saying the chances are good, maybe 70% or 80%, but you're leaving room for other possibilities.

If dark clouds roll in and the air feels heavy, you might say it will probably rain this afternoon. You're making an educated guess based on what you observe, not just a wild hope. When your friend says she'll probably come to your birthday party, she means the odds are in your favor, though something unexpected could change her plans.

The word sits in an interesting middle ground. It's more confident than maybe or possibly, but less certain than definitely or surely. Understanding this difference helps you communicate clearly. If you tell your parents you probably finished your homework, they'll want to check. If you say you definitely finished it, they'll trust you completely.

People use probably when they have good reasons to believe something without having absolute proof. Scientists say dinosaurs probably had feathers based on fossil evidence. Historians say the ancient library of Alexandria probably held hundreds of thousands of scrolls, though exact records don't survive. In each case, the evidence points strongly in one direction, but some uncertainty remains.