choropleth map
Concise Oxford English Dictionary © 2008 Oxford University Press:
choropleth map /ˈkɒrə(ʊ)plɛθ/
▶noun a map which uses shading, colouring, or symbols to show the average values of a particular quantity in different areas.
– origin 1930s: from Gk khōra ‘region’ + plēthos ‘multitude’.
'choropleth map' also found in these Oxford entries:

