transition zone

from the GNU version of the Collaborative International Dictionary of English.

• (Biogeography) The zone lying between the Boreal and Sonoran zones of North America. It includes an eastern or humid subdivision and a western arid one of corresponding temperature comprising the northern Great Plains and the lower slopes of the mountains of the western United States and Mexico. Called also Neutral zone.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several physical zones in which the properties, or the behaviour of something undergoes a radical change