Definition: a grilled sandwich of corned beef, Swiss cheese, and sauerkraut usually on rye bread
There are two commonly repeated stories of how the Reuben came to have its name. Each involves a man named Reuben and cabbage. One hinges on a hungry costar of Charlie Chaplin's in a New York restaurant, and the other involves a late-night snack during an Omaha poker game.
Merriam-Webster's etymology favors the one with the poker game: the Reuben probably gets its name from Reuben Kulakofsky, an American grocer. According to the lore, Kulakofsky's hunger during a poker game in the mid-late 1920s led either to him making the sandwich now known as a Reuben, or to him requesting a sandwich with corned beef and sauerkraut, inspiring the chef of the restaurant hosting the poker game to create the famed sandwich. The granddaughter of that alleged chef recently recounted her family's version of the story in the New York Times, reporting that the original sandwich included corned beef, Swiss cheese, and sauerkraut mixed with Thousand Island dressing on dark rye, then grilled.
The other story traces the Reuben to the famous "Reuben's Restaurant" in New York City, and an actress named Annette Seelos, who was filming a movie with Charlie Chaplin at the time. Seelos was famished one night in 1914 and was given an improvised sandwich of ham, turkey, Swiss cheese, cole slaw, and Russian dressing on rye, which became an instant success.
The word "probably" in our etymology leaves open the possibility that evidence will emerge that tips the balance in favor of the Reuben's Restaurant origin, but based on the existing menu evidence, we consider the Omaha origin to be the more plausible at this time.