The Free On-line Dictionary of Computing (30 December 2018):
A programmer who works on a fixed-length or
temporary contract, and is often employed to write certain types
of code or to work on a specific project.
Despite the fact that contractors usually cost more than hiring a
permanent employee with the same skills, it is common for
organisations to employ them for extended periods, sometimes
renewing their contracts for many years, due to lack of certainty
about the future or simple lack of planning.
A contract programmer may be independent or they may work in a
supplier's professional services department, providing
consultancy and programming services for the supplier's products.