A diner is a small, inexpensive restaurant found across the United States, as well as in Canada and parts of Western Europe. Diners offer a wide range of foods, mostly American cuisine, a casual atmosphere, and, characteristically, a combination of booths served by a waitstaff and a long sit-down counter with direct service, in the smallest simply by a cook. Many diners have extended hours, and some along highways and areas with significant shift work stay open for 24 hours.

Considered quintessentially American, many diners share an archetypal exterior form. Some of the earliest were converted rail cars, retaining their streamlined structure and interior fittings. From the 1920s to the 1940s, diners, by then commonly known as "lunch cars", were usually prefabricated in factories, like modern mobile homes, and delivered on site with only the utilities needing to be connected. As a result, many early diners were typically small and narrow to fit onto a rail car or truck. This small footprint also allowed them to be fitted into tiny and relatively inexpensive lots that otherwise were unable to support a larger enterprise. Diners were historically small businesses operated by the owner, with some presence of restaurant chains evolving over time.

Diners typically serve staples of American cuisine such as hamburgers, french fries, club sandwiches, and other simple, quickly cooked, and inexpensive fare, such as meatloaf or steak. Much of the food is grilled, as early diners were based around a gas-fueled flattop grill. Coffee is a diner staple. Diners often serve milkshakes and desserts such as pies, cake or ice cream. Comfort food cuisine draws heavily from, and is deeply rooted in, traditional diner fare. Along with greasy spoon menu items, many diners will serve regional cuisine as well, such as clam chowder in New England and tacos in California.

Classic American diners often have an exterior layer of stainless steel siding—a feature unique to diner architecture. In some cases, diners share nostalgic, retro-style features also found in some restored drive-ins and old movie theatres.