Portobello mushrooms, scientifically known as Agaricus bisporus, are large and meaty mushrooms that are mature versions of the common button mushroom. They are characterized by their broad, flat caps, which can measure up to six inches in diameter, and their thick, sturdy stems. Portobellos have a rich, earthy flavor and a dense, chewy texture, making them a popular choice for grilling, roasting, and stuffing. These mushrooms are versatile and can be used as a meat substitute in vegetarian dishes or enjoyed as a hearty addition to various culinary creations.