A dialect is a sub-division of a language used by a group of people who share certain common non-linguistic features. It’ s hard to give a definition that can definitely distinguish dialect from language, for the difference between them is not absolute. In other words, any rigid definition is not accurate. 

It appears to be clear that British English and American English are two dialects of the same language-English. Like British English, various kinds of English applied by different countries are different dialects of English. But for the sake of convenience, they are all simply called as English. 

However, variations in some languages are neither associated with social groups nor connected with individuals, but only with situations where they are used. Such variations are called registers, whose importance has finally been noticed by us. The study of register can be regarded as the study of language in the context of using language.