transition zone

Short definition from Wiktionary

Noun

1. 1
   Any of several physical zones in which the properties, or the behaviour of something undergoes a radical change: The stage in the flow or a fluid during which it changes from laminar to turbulent
2. 2
   Any of several physical zones in which the properties, or the behaviour of something undergoes a radical change: The region between the upper and lower mantle characterized by a rapid increase in density
3. 3
   Any of several physical zones in which the properties, or the behaviour of something undergoes a radical change: The area between a central business district and residential housing in a city