Prairies are ecosystems considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and a composition of grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type. Temperate grassland regions include the Pampas of Argentina, Brazil and Uruguay, and the steppe of Romania, Ukraine, Russia, and Kazakhstan. Lands typically referred to as "prairie" (a French loan word) tend to be in North America. The term encompasses the lower and mid-latitude of the area referred to as the Interior Plains of Canada, the United States, and Mexico. It includes all of the Great Plains as well as the wetter, hillier land to the east. From west to east, generally the drier expanse of shortgrass prairie gives way to mixed grass prairie and ultimately the richer and wetter soils of the tallgrass prairie.

In the U.S., the area is constituted by most or all of the states, from north to south, of North Dakota, South Dakota, Nebraska, Kansas, and Oklahoma, and sizable parts of the states of Montana, Wyoming, Colorado, New Mexico, Texas in the west, and to the east, Minnesota, Wisconsin, Iowa, Missouri, Illinois, and Indiana. The Palouse of Washington and the Central Valley of California are also prairies. The Canadian Prairies occupy vast areas of Manitoba, Saskatchewan, and Alberta. Prairies may contain various lush flora and fauna, often contain rich soil maintained by biodiversity, with a temperate climate and a varied view.

According to Theodore Roosevelt:

We have taken into our language the word prairie, because when our backwoodsmen first reached the land [in the Midwest] and saw the great natural meadows of long grass—sights unknown to the gloomy forests wherein they had always dwelt—they knew not what to call them, and borrowed the term already in use among the French inhabitants.

Prairie (pronounced [pʁɛʁi]) is the French word for "meadow", formed ultimately from the Latin root word pratum (which has the same meaning).

The formation of the Canadian Prairies started with the uplift of the Rocky Mountains near Alberta. The mountains created a rain shadow which resulted in lower precipitation rates downwind.

The parent material of most prairie soil was distributed during the last glacial advance that began about 110,000 years ago. The glaciers expanding southward scraped the landscape, picking up geologic material and leveling the terrain. As the glaciers retreated about 10,000 years ago, they deposited this material in the form of till. Wind-based loess deposits also form an important parent material for prairie soils.

Tallgrass prairie evolved over tens of thousands of years with the disturbances of grazing and fire. Native ungulates such as bison, elk, and white-tailed deer roamed the expansive, diverse grasslands before European colonization of the Americas. For 10,000-20,000 years, native people used fire annually as a tool to assist in hunting, transportation, and safety. Evidence of ignition sources of fire in the tall grass prairie are overwhelmingly human as opposed to lightning. Humans, and grazing animals, were active participants in the process of prairie formation and the establishment of the diversity of graminoid and forbs species. Fire has the effect on prairies of removing trees, clearing dead plant matter, and changing the availability of certain nutrients in the soil from the ash produced. Fire kills the vascular tissue of trees, but not prairie species, as up to 75% (depending on the species) of the total plant biomass is below the soil surface and will re-grow from its deep (upwards of 20 feet) roots. Without disturbance, trees will encroach on a grassland and cast shade, which suppresses the understory. Prairie and widely spaced oak trees evolved to coexist in the oak savanna ecosystem.