A diptych (from the Greek di "two" and ptychē "fold") is a pair of paitings. You can also say triptych for a group of three paintings belonging together, septych for a group of seven and so on. Polyptych is an umbrella term for these words.
Is there a word for a single painting that has similar etymology?
If not, is there a word for a single work of art that can be used when one wants to emphasize that it's a single independent work of art, as opposed to a part of a group (for example a diptych)?