A vegetation formation characterized by woody plants of low stature (3.3–10 ft or 1–3 m tall), impenetrable because of tough, rigid, interlacing branches, with small, simple, waxy, evergreen, thick leaves. The term refers to evergreen oak, Spanish chapparo, and therefore is uniquely southwestern North American. This type of vegetation has its center in California and occurs continuously over wide areas of mountainous to sloping topography. The Old World Mediterranean equivalent is called maquis or macchie, with nomenclatural and ecological variants in the countries from Spain to the Balkans. Physiognomically similar vegetation occurs also in South Africa, Chile, and southwestern Australia in areas of Mediterranean climates, that is, with very warm, dry summers and maximum precipitation during the cool season. The floras of these five areas with Mediterranean climates are altogether different.

The characteristic species of the true chaparral of California include Adenostema fasciculatum, Ceanothus cuneatus, Quercus dumosa, Heteromeles arbutifolia, Rhamnus californica, R. crocea, and Cercocarpus betuloides, plus a host of endemic species of Arctostaphylos and Ceanothus and other Californian endemics, both shrubby and herbaceous. These plants determine the formation's physiognomy. It is a dense, uniform-appearing, evergreen, shrubby cover with sclerophyllous leaves and deep-penetrating roots.

Ecologically, chaparral occurs in a climate which is hot and dry in summer, cool but not much below freezing in winter, with little or no snow, and with excessive winter precipitation that leaches the soil of nutrients. The need for water and its supply are exactly out of phase.

Chaparral soils are generally rocky, often shallow, or of extreme chemistry such as those derived from serpentine, and are always low in fertility. In the very precipitous southern Californian mountains, soil erosion rates may be 0.04 in. (1 mm) per year over large watershed areas.