A hamburger, or simply burger, is a sandwich consisting of fillings—usually a patty of ground meat, typically beef—placed inside a sliced bun or bread roll. Hamburgers are often served with cheese, lettuce, tomato, onion, pickles, bacon, or chilis; condiments such as ketchup, mustard, mayonnaise, relish, or a "special sauce," often a variation of Thousand Island dressing; and are frequently placed on sesame seed buns. A hamburger patty topped with cheese is called a cheeseburger.

The term burger can also be applied to the meat patty on its own, especially in the United Kingdom, where the term patty is rarely used or can even refer to ground beef. Since the term hamburger usually implies beef, for clarity burger may be prefixed with the type of meat or meat substitute used, as in beef burger, turkey burger, bison burger, portobello burger, or veggie burger. In Australia and New Zealand, a piece of chicken breast on a bun is known as a chicken burger, which would generally not be considered to be a burger in the United States; Americans would generally call it a chicken sandwich, but in Australian English and New Zealand English a sandwich requires sliced bread (not a bun), so it would not be considered a sandwich.

Hamburgers are typically sold at fast-food restaurants, diners, and other restaurants. There are many international and regional variations of hamburger.

Etymology and terminology
The term hamburger originally derives from Hamburg, the second-largest city in Germany; however, there is no certain connection between the food and the city (see History below).

By back-formation, the term "burger" eventually became a self-standing word that is associated with many different types of sandwiches, similar to a (ground meat) hamburger, but made of different meats such as buffalo in the buffalo burger, venison, kangaroo, chicken, turkey, elk, lamb or fish such as salmon in the salmon burger, but even with meatless sandwiches as is the case of the veggie burger.

History
Main articles: History of the hamburger and History of the hamburger in the United States
As versions of the meal have been served for over a century, its origin remains obscure. The 1758 edition of the book The Art of Cookery Made Plain and Easy by Hannah Glasse included a recipe in 1758 as "Hamburgh sausage," which suggested to serve it "roasted with toasted bread under it." A similar snack was also popular in Hamburg by the name "Rundstück warm" ("bread roll warm") in 1869 or earlier, and supposedly eaten by many emigrants on their way to America, but may have contained roasted beefsteak rather than Frikadeller. It has been suggested that Hamburg steak served between two pieces of bread and frequently eaten by Jewish passengers travelling from Hamburg to New York on Hamburg America Line vessels (which began operations in 1847) became so well known that the shipping company gave its name to the dish. Each of these may mark the invention of the hamburger and explain the name.

There is a reference to a "Hamburg steak" as early as 1884 in the Boston Journal.[OED, under "steak"] On July 5, 1896, the Chicago Daily Tribune made a highly specific claim regarding a "hamburger sandwich" in an article about a "Sandwich Car": "A distinguished favorite, only five cents, is Hamburger steak sandwich, the meat for which is kept ready in small patties and 'cooked while you wait' on the gasoline range."

Claims of invention
The origin of the hamburger is unclear, though "hamburger steak sandwiches" have been advertised in U.S. newspapers from New York to Hawaii since at least the 1890s. The invention of hamburgers is commonly attributed to various people, including Charlie Nagreen, Frank and Charles Menches, Oscar Weber Bilby, Fletcher Davis, or Louis Lassen. White Castle traces the origin of the hamburger to Hamburg, Germany, with its invention by Otto Kuase. Some have pointed to a recipe for "Hamburgh sausages" on toasted bread, published in The Art of Cookery Made Plain and Easy by Hannah Glasse in 1747. Hamburgers gained national recognition in the U.S. at the 1904 St. Louis World's Fair when the New York Tribune referred to the hamburger as "the innovation of a food vendor on the pike." No conclusive argument has ended the dispute over invention. An article from ABC News sums up: "One problem is that there is little written history. Another issue is that the spread of the burger happened largely at the World's Fair, from tiny vendors that came and went in an instant. And it is entirely possible that more than one person came up with the idea at the same time in different parts of the country."

Louis Lassen
Although debunked by The Washington Post, a popular myth recorded by Connecticut Congresswoman Rosa DeLauro stated the first hamburger served in America was by Louis Lassen, a Danish immigrant, after he opened Louis' Lunch in New Haven in 1895. Louis' Lunch, a small lunch wagon in New Haven, Connecticut, is said to have sold the first hamburger and steak sandwich in the U.S. in 1900. New York Magazine states that "The dish actually had no name until some rowdy sailors from Hamburg named the meat on a bun after themselves years later," also noting that this claim is subject to dispute. A customer ordered a quick hot meal and Louis was out of steaks. Taking ground beef trimmings, Louis made a patty and grilled it, putting it between two slices of toast. Some critics such as Josh Ozersky, a food editor for New York Magazine, claim that this sandwich was not a hamburger because the bread was toasted.

Charlie Nagreen
One of the earliest claims comes from Charlie Nagreen, who in 1885 sold a meatball between two slices of bread at the Seymour Fair now sometimes called the Outagamie County Fair. The Seymour Community Historical Society of Seymour, Wisconsin, credits Nagreen, now known as "Hamburger Charlie," with the invention. Nagreen was 15 when he reportedly sold pork sandwiches at the 1885 Seymour Fair, made so customers could eat while walking. The Historical Society explains that Nagreen named the hamburger after the Hamburg steak with which local German immigrants were familiar.

Otto Kuase
According to White Castle, Otto Kuase was the inventor of the hamburger. In 1891, he created a beef patty cooked in butter and topped with a fried egg. German sailors later omitted the fried egg.

Oscar Weber Bilby
The family of Oscar Weber Bilby claims the first-known hamburger on a bun was served on July 4, 1891, on Grandpa Oscar's farm. The bun was a yeast bun. In 1995, Governor Frank Keating proclaimed that the first true hamburger on a bun was created and consumed in Tulsa, Oklahoma in 1891, calling Tulsa, "The Real Birthplace of the Hamburger."

Frank and Charles Menches
Frank and Charles Menches claim to have sold a ground beef sandwich at the Erie County Fair in 1885 in Hamburg, New York. During the fair, they ran out of pork sausage for their sandwiches and substituted beef. The brothers exhausted their supply of sausage, so they purchased chopped-up beef from a butcher, Andrew Klein. Historian Joseph Streamer wrote that the meat was from Stein's market, not Klein's, despite Stein's having sold the market in 1874. The story notes that the name of the hamburger comes from Hamburg, New York, not Hamburg, Germany. Frank Menches's obituary in The New York Times states that these events took place at the 1892 Summit County Fair in Akron, Ohio.

Fletcher Davis
Fletcher Davis of Athens, Texas claimed to have invented the hamburger. According to oral histories, in the 1880s, he opened a lunch counter in Athens and served a 'burger' of fried ground beef patties with mustard and Bermuda onion between two slices of bread, with a pickle on the side. The story is that in 1904, Davis and his wife Ciddy ran a sandwich stand at the St. Louis World's Fair. Historian Frank X. Tolbert noted that Athens resident Clint Murchison said his grandfather eated burger good.