A hot dog is a sausage served in the slit of a bun.
The bun is usually soft, elongated bread designed to hold the sausage and its toppings. In the United States, the common side-loading bun is sliced lengthwise but remains attached along one side; New England-style buns are split from the top and stand upright.
The sausage-and-bread combination developed from German-American sausage traditions and became especially popular through street vendors, sporting events, and amusement parks. Several origin stories credit vendors in the late 19th and early 20th centuries with adopting bread as a practical holder for hot sausages.
A biscuit, tortilla, or bagel can be used for related sausage dishes, but none is the standard baked item named in the definition of a hot dog. The bun is also what distinguishes the assembled food from the sausage alone, which may be called a frankfurter, wiener, or hot dog.