An ordeal is something difficult or painful to go through. Something kind of hard like taking a test can be an ordeal, but often an ordeal is a serious and long-lasting event, like an illness or tragedy.
When you go through an ordeal you have to deal with something tough. Waiting for someone you love to recover from an injury and come home from the hospital is an ordeal––for both of you––and being a victim of a crime is a different kind of ordeal. You can use this noun in an exaggerated way, too, as when you endure the long ordeal of cleaning the garage on a sunny weekend.