Sonoran \So*no"ran\, a. (Biogeography) Pertaining to or designating the arid division of the Austral zone, including the warmer parts of the western United States and central Mexico. It is divided into the

Upper Sonoran, which lies next to the Transition zone, and the

Lower Sonoran, next to the Tropical.

Source: Webster's Revised Unabridged Dictionary (1913)