A diptych is a pair of paintings. What is a single painting called?

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)?

Solution 1:

The etymology of diptych and its extensions triptych and polyptych includes the concept of the folding of the images.

Mid 19th century: from late Latin polyptycha (neuter plural) 'registers', from Greek poluptukhos 'having many folds', from polu- 'many' + ptukhē 'fold'.

ODO

As Chasly has commented, the concept of folding a single panel is rather self-defeating, so monoptych, which would be etymologically correct, is not found.

Generally, as the linked paintings are painted on board, the term panel is used.