Correct term to describe an unpublished research paper?

Solution 1:

A research paper that has been submitted for publication, but not yet approved by peer review, is called a preprint. If the article has passed review and been accepted for publication, but not yet actually published, then it's a postprint. Note that these are terms that would be well-known within academia or research labs, but not necessarily well-known in the commercial world.

In some fields, preprints are widely read and available in collections called "preprint archives." One of the oldest and most famous ones is called arXiv and was one of the first large sites on the World Wide Web (which it actually predates).