A polymath is a person who knows a lot about a lot of subjects. If your friend is not only a brilliant physics student but has also published a poetry collection and won prizes at political debates, you can describe her as a polymath.
You can think of a polymath as a classic "Renaissance man." Imagine Leonardo da Vinci, for example, who was not only an amazing artist, but also an engineer, inventor, mathematician, and much more. When a person's knowledge covers many different areas, he or she is a polymath. The Greek word for it is polymathes, "having learned much," with poly meaning "much," and manthanein meaning "learn."