a prolonged and very unpleasant experience, or a severe trial;
e.g. It would be quite an ordeal to travel all that distance for such a small task.
(noun)
in historical usage, a primitive test of guilt (a means of determining innocence) that involves subjecting the accused to dangerous tests or severe pain, often believed to be under supernatural control, in which survival proves innocence;
e.g. She was subjected to an ordeal by fire to prove her innocence.