Branches of linguistics. Synchronic vs diachronic approaches to the language study. Lexicology – ‘the science of the word’

Commonly studied concepts in dialectology include:
-the problem of mutual intelligibility in defining languages and dialects;
-situations of diglossia, where two dialects are used for different functions;
-ialect continuum, i.e. a network of dialects in which geographically adjacent dialects are mutually comprehensible, but with comprehensibility steadily decreasing as distance between the dialects increases, e.g. Dutch-German dialect continuum, a vast network of dialects with two recognized literary standards;
-pluricentrism, where what is essentially a single genetic language exists as two or more standard varieties.
Basic Notions
According to the New English Dictionary, the oldest sense of the term dialect (1577) was ‘a manner of speaking’ or ‘phraseology’, in accordance with its derivation from the Gr. dialectos ‘a discourse or way of speaking’. The modern meaning is somewhat more precise. In relation to a language such as English, the term dialect is used in a special sense to signify ‘a local variety of speech differing from the standard or literary language’.
A dialect is a linguistic system that
-shows a high degree of similarity to other systems so that at least partial mutual intelligibility is possible;
-is tied to a specific region in such a way that the regional distribution of the system does not overlap with an area covered by another such system;
-does not have a written or standardised form, i.e. does not have officially standardised orthographic and grammatical rules.
A regional accent refers to features of pronunciation against a geographical background. Accents may also convey social implications and be prestigious, neutral or low class.
A regional dialect refers to features of grammar and vocabulary against a geographical background. A regional dialect includes a distinctive regional accent but the reverse does not necessarily follow.