Portobello is a type of mushroom that belongs to the genus Aureolus. These are large, meaty mushrooms with a cap diameter ranging from 3-10 inches and can grow up to 2 feet in length. They have a thick stem and a distinctive flavor that makes them popular for cooking. The name "portobello" is believed to be derived from the Italian city of Portobello, near Venice, where these mushrooms were first introduced to the United States in the early 20th century.