To confound means to confuse or puzzle someone completely, leaving them unable to understand or figure something out. When a magician performs an impossible-seeming trick, she confounds her audience because they can't work out how she did it. A difficult riddle might confound you until someone explains the clever twist.

The word suggests more than mild confusion: when something confounds you, it really stumps you. A detective might be confounded by a mystery with no logical explanation. A scientist could be confounded by experimental results that contradict everything she expected. Teachers are sometimes confounded when a normally struggling student suddenly aces a test, or when a top student suddenly fails one.

When something confounds expectations, it does the opposite of what everyone predicted. If the underdog team wins the championship, they've confounded all the experts who said they had no chance. The word can also describe mixing things up so they're hard to separate: a researcher might worry that two factors are confounded in an experiment, making it impossible to tell which one caused the results.