A columnist writes a column: a piece that appears regularly in a newspaper or magazine, often on one subject, and often in the writer’s own voice. A reporter tries to tell you what happened. A columnist is allowed — even expected — to argue, joke, or take a side. There are sports columnists, political columnists, advice columnists. The word is ordinary news language, not slang, and the job can feel quite public because people know the name. Though actually, column here is the strip of writing on the page, not a stone pillar. Same English word, two pictures.