A subdialect is a subdivision of dialect. Subdialects can be divided further, ultimately down to idiolects.
Normally subdialects of one dialect are quite close to each other, differing mainly in pronunciation and certain local words.
In some languages there is a special word for such a small variety of language, ex. Russian: говор govor, Romanian: grai, Polish: gwara.
References
Subdialect Wikipedia(Text) CC BY-SA