[dragonwriter]

Linguists tend toward using mutual intelligibility as the difference between a language and a dialect, but it's fuzzy. Beijing and Shanghai dialect are almost totally mutually unintelligible, but if you walked from Beijing down to Shanghai, each village, town, or city you pass will speak a dialect that's still mutually intelligible with the immediately neighboring village/town/city's.

So, its a "ring language" (by analogy to a "ring species" [1] in biology.)

http://en.wikipedia.org/wiki/Ring_species

[abayer]

The equivalent term in linguistics is "dialect continuum" - http://en.wikipedia.org/wiki/Dialect_continuum