A "contributing columnist" is a writer who provides regular articles or opinion pieces for a publication, but is not an official staff member. They contribute their writing on a specific topic or area of expertise. Full definition
She is currently a freelance contributing columnist for the Star. (thestar.com)
New York Times bestseller and author of seven books, Pam Anderson is AARP's official food expert and a Runner's World magazine contributing columnist. (californiaavocado.com)