Classical language
A classical language is a language with a literary tradition that can be judged as "classical".
According to George L. Hart:
[To] qualify as a classical tradition, a language must fit several criteria: it should be ancient, it should be an independent tradition that arose mostly on its own not as an offshoot of another tradition, and it must have a large and extremely rich body of ancient literature.
Note that the judgement of a language as "classical" is a judgement of its literature, not the language itself. Although languages may change at different rates, languages that change at slower rates are not necessarily the same as languages judged as "classical" or "ancient". Also, since there is no language immune from change, classical languages tend to be either dead languages, or show a high degree of diglossia, as the spoken varieties of the language diverge further and further away from the classical written language over centuries.