One of the FAQ
s about natural language
: what is the difference between a dialect
and a language
A dialect is a (usually: regional) variant of a language. The subject of study of linguistics is language as it occurs, and it so happens that language occurs in a continuous and always evolving range of varieties, often without clear borders. A nice example is German: one can travel from village to village, starting near Kortrijk or near Amsterdam, all the way to Szczecin, Vienna or Chur, and see only very small changes in the local dialect along the way, while over larger distances the dialects can no longer be mutually understood - actually, some are not known as dialects of German, but of Dutch, another well-established standard language within the same dialect continuum.
If a language is widely used, standard, 'official' forms will develop, with normative rules on spelling, pronunciation, grammar, and vocabulary. The term 'a language' is often used to refer to such a standardized form of language, mainly by non-linguists.
It is rarely by its linguistic properties that a particular language obtains recognition as 'a language'.
Rather, it is a question of official backing.
So you shouldn't really ask this question to a linguist; ask a historian instead!
For example, the dialect continuum mentioned above covers the Netherlands, half of Belgium, Luxemburg, Germany, more than half of Switzerland, Austria, a region in Italy, and some small regions elsewhere; national interdialectal standards are developing in each of these states, but the two traditionally recognised languages are Dutch (in the Netherlands and most of the Belgian area) and German (elsewhere). They have both developed clear identities through centuries of teaching and usage. From a linguist's point of view however, they are just two variants that happen to have wider national use and recognition than any of the regional dialects.
This point of view is summarized in the well-known saying
A shprakh iz a diyalekt mit an armey un a flot.
("A language is a dialect with an army and a navy.")
This quote's earliest attested appearance, in Yiddish, is in an article by Max Weinreich (1945); earlier it appeared without the "and a navy" part. (Note that Switzerland and Austria do not border on the sea - they do not have navies.)
See http://www.edu-cyberpg.com/Linguistics/armynavy.html for details.