A sandwich is a dish typically consisting variously of meat, cheese, sauces, and vegetables used as a filling between slices of bread, or placed atop a slice of bread; or, more generally, any dish in which bread serves as a container or wrapper for another food type. The sandwich began as a portable, convenient food in the Western world, and over time it has become prevalent worldwide.

There has been social media debate over the precise definition of sandwich, specifically whether a hot dog or open sandwich can be categorised as such. Other items, like hamburgers and burritos, were also considered. In the United States, the Department of Agriculture (USDA) and the Food and Drug Administration (FDA) are the responsible agencies for protecting the definition of sandwich. The USDA uses the definition, "at least 35% cooked meat and no more than 50% bread" for closed sandwiches, and "at least 50% cooked meat" for open sandwiches. However, the same USDA manual determines that burritos and fajitas are "sandwich-like" and frankfurters are "sandwich type", while stromboli is explicitly excluded. In Britain, the British Sandwich Association defines a sandwich as "any form of bread with a filling, generally assembled cold"; a definition which includes wraps and bagels, but potentially excludes dishes assembled and served hot, such as burgers.

Sandwiches are a popular type of lunch food, taken to work, school, or picnics to be eaten as part of a packed lunch. The bread is frequently coated with condiments such as mayonnaise or mustard to enhance its flavour and texture, but may be served plain ("dry"). As well as being homemade, sandwiches are also widely sold in various retail outlets and can be served hot or cold. Although savoury sandwiches—such as deli meat sandwiches—are in the majority, sweet sandwiches—such as jam sandwiches and fluffernutters—form their own category.

The sandwich is named after the inventor of a certain roast-beef sandwich in 18th-century England, John Montagu, 4th Earl of Sandwich. The Wall Street Journal has described it as Britain's "biggest contribution to gastronomy".

The use of some kind of bread or bread-like substance to lie under some other food, or to scoop it up and enclose or wrap it, is found in many cultures historically. Before being known as a "sandwich", this food combination seems to have been known as "bread and meat" or "bread and cheese". These two phrases are found throughout English drama from the 16th and 17th centuries.

In the first century BC the Jewish sage Hillel the Elder is said to have wrapped meat from the Paschal lamb and bitter herbs in a soft matzah—flat, unleavened bread—during Passover in the manner of a modern wrap made with flatbread. Flat breads of only slightly varying kinds have long been used to scoop or wrap small amounts of food en route from platter to mouth throughout Western Asia and North Africa. From the cuisines of Morocco to Ethiopia to India, bread is usually baked in flat rounds, contrasting with the European loaf tradition.

During the Middle Ages in Europe, thick slabs of coarse and usually stale bread, called "trenchers," were used as plates. After a meal, the food-soaked trencher was fed to a dog or to beggars at the tables of the wealthy, and eaten by diners in more modest circumstances. The immediate culinary precursor with a direct connection to the English sandwich was to be found in the Netherlands of the seventeenth century, where the naturalist John Ray observed that in the taverns beef hung from the rafters "which they cut into thin slices and eat with bread and butter laying the slices upon the butter"—explanatory specifications that reveal the Dutch belegde broodje, open-faced sandwich, was as yet unfamiliar in England. The word "sandwich" appears, referring to a certain roast beef sandwich in England.

Initially perceived as food that men shared while gaming and drinking at night, the sandwich slowly began appearing in polite society as a late-night meal among the aristocracy. The sandwich is named after John Montagu, 4th Earl of Sandwich, an eighteenth-century English aristocrat. It is commonly said that Lord Sandwich, during long sessions of cribbage and other card games at public gambling houses, would order his valet to bring him roast beef between two pieces of toasted bread. He was fond of this form of food because it allowed him to continue gambling while eating, without the need for a fork, and without getting his cards greasy from eating meat with his bare hands. The dish then grew in popularity in London, and Sandwich's name became associated with it. The rumour in its familiar form appeared in Pierre-Jean Grosley's Londres (Neuchâtel, 1770), translated as A Tour to London in 1772; Grosley's impressions had been formed during a year in London in 1765. An alternative is provided by Sandwich's biographer Nicholas A. M. Rodger, who suggests Sandwich's commitments to the Royal Navy, politics and the arts mean the first sandwich was more likely to have been consumed at his desk.