Grass is the common word that generally describes monocotyledonous green plants. The family Poaceae (or Gramineae) are the "true grasses" and include most plants grown as grains, for pasture, and for lawns (turf). They include some more specialised crops such as lemongrass, as well as many ornamental plants. They also include plants often recognized to be grasses, such as bamboos, maize or some species of weeds called crab grass.

The term grass can refer to family Poaceae, the 'true grasses', or to grass-like plants which includes the Poaceae and typically also the rushes (Juncaceae) and sedges (Cyperaceae), which somewhat resemble grass. Another, more specific, term for the latter group is graminoid.

Grass may also be used to describe completely unrelated plants, sometimes of similar appearances to grass, with leaves rising vertically from the ground, and sometimes of dissimilar appearance. The term came about in the early 15th century, from the Old English græs, derived from the same root as "grow".