An Op-Ed is a piece of writing expressing an opinion or editorial. Such items are often found in a full newspaper page, containing such articles by columnists, letters to the editor, and other points, rather than news or facts. It is primarily an American term.
The name came from the tradition of placing such material on the page opposite to the editorial page; deriving it from Op-inion and Ed-itorial is a fake etymology.
Most op-ed pieces take the form of an essay or thesis, using arguments to promote a point of view. Such a point of view is usually in line with the newspaper's editorial slant, though dissenting opinions are often given space to promote discussion.
Op-Ed has become a general category to identify opinion from fact regardless of the medium. For example, web pages containing opinion articles are labeled Op-Ed even though the original meaning is not relevant.