very small pieces of beef (= meat from cattle), or this meat pressed into a round, flat shape, cooked, and usually eaten as a sandwich in a small, round roll (= piece of bread): [ U ] a pound of hamburger (meat) [ C ] I’ll take a hamburger and French fries, please.

The word really comes from the name of the German city Hamburg. Hamburger, when capitalized, means "of Hamburg." Cakes of ground beef, often served raw, were a popular food in northern Germany in the 19th century, and so they became known in English as Hamburger steaks. The name was later shortened to hamburger.