A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever having used actual seeds or cereal grains. Rather, expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas, essentially the premodern equivalent of legal boilerplate.

The fundamental unit of the troy, apothecary and avoirdupois systems, commonly known as the grain (or less commonly the barleycorn), is nominally based on the grain of barley. The fundamental unit of the pre-1527 English weight system for Canada, known as Tower weights, was a different sort of grain known as the wheat-grain.

The grain is the only unit of mass measure common to the three traditional English mass and weight systems (avoirdupois, Apothecaries', Troy); the obsolete Tower grain was, by definition, exactly ⁴⁵⁄₆₄ of a troy grain.

Cereals are grasses (members of the monocot family Poaceae, also known as Gramineae) cultivated for the edible components of their grain (botanically, a type of fruit called a caryopsis), composed of the endosperm, germ, and bran. Cereal grains are grown in greater quantities and provide more food energy worldwide than any other type of crop^{[citation needed]}; they are therefore staple crops.

In their natural form (as in whole grain), they are a rich source of vitamins, minerals, carbohydrates, fats, oils, and protein. However, when refined by the removal of the bran and germ, the remaining endosperm is mostly carbohydrate and lacks the majority of the other nutrients. In some developing nations, grain in the form of rice, wheat, millet, or maize constitutes a majority of daily sustenance. In developed nations, cereal consumption is moderate and varied but still substantial.

The word cereal derives from Ceres, the name of the Roman goddess of harvest and agriculture.

The first cereal grains were domesticated about 12,000 years ago by ancient farming communities in the Fertile Crescent region. Emmer wheat, einkorn wheat, and barley were three of the so-called Neolithic founder crops in the development of agriculture.

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