Poaceae (/poʊˈeɪsiaɪ/) or Gramineae is a large and nearly ubiquitous family of monocotyledonous flowering plants known as grasses, commonly referred to collectively as grass. Poaceae includes the cereal grasses, bamboos and the grasses of natural grassland and cultivated lawns and pasture.

The Poaceae are the most important source of the world's dietary energy supply. They provide, through direct human consumption, just over one-half (51%) of all dietary energy; rice provides 20%, wheat supplies 20%, maize (corn) 5.5%, and other grains 6%.

Grasses have stems that are hollow except at the nodes and narrow alternate leaves borne in two ranks. The lower part of each leaf encloses the stem, forming a leaf-sheath. With around 780 genera and around 12,000 species, Poaceae are the fifth-largest plant family, following the Asteraceae, Orchidaceae, Fabaceae and Rubiaceae.

Grasslands such as savannah and prairie where grasses are dominant are estimated to constitute 40.5% of the land area of the Earth, excluding Greenland and Antarctica. Grasses are also an important part of the vegetation in many other habitats, including wetlands, forests and tundra.

The Poaceae are the most economically important plant family, providing staple foods from domesticated cereal crops such as maize, wheat, rice, barley, and millet as well as feed for meat-producing animals. They are used as building materials (bamboo, thatch, and straw). They are a source of biofuel, primarily via the conversion of maize to ethanol.

Though they are commonly called "grasses", seagrasses, rushes, and sedges fall outside this family. The rushes and sedges are related to the Poaceae, being members of the order Poales, but the seagrasses are members of order Alismatales.

Grasses are the dominant vegetation in many habitats, including grassland, salt-marsh, reedswamp and steppes. They also occur as a smaller part of the vegetation in almost every other terrestrial habitat.  Grass-dominated biomes are called grasslands. If only large, contiguous areas of grasslands are counted, these biomes cover 31% of the planet's land. Grasslands include pampas, steppes, and prairies. Grasses provide food to many grazing mammals—such as livestock, deer, and elephants—as well as to many species of butterflies and moths.  Many types of animals eat grass as their main source of food, and are called graminivores – these include cattle, sheep, horses, rabbits and many invertebrates, such as grasshoppers and the caterpillars of many brown butterflies. Grasses are also eaten by omnivorous or even occasionally by primarily carnivorous animals.

Grasses are unusual in that the meristem is located near the bottom of the plant; hence, they can quickly recover from cropping at the top. The evolution of large grazing animals in the Cenozoic contributed to the spread of grasses. Without large grazers, fire-cleared areas are quickly colonized by grasses, and with enough rain, tree seedlings. Trees eventually outcompete most grasses. Trampling grazers kill seedling trees but not grasses.

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