Nevertheless, it remains the case that although spoken American and British English are generally mutually intelligible, there are enough differences to cause occasional misunderstandings or at times embarrassment—for example, some words that are quite innocent in one dialect may be considered vulgar in the other.
II. Historical Background of English

2.1 The definition of British English
   The definition of British English: British English, mainly existing in the British Isles of England by the English rule, is the official language of British and Commonwealth countries.
2.2 The definition of American English
The definition of American English: American English in the form of a USA use, American English is a separate branch in British English, or as the linguists say, American English is a land in America that has been used in British English (transplant language).
2.3 Review of The Development of English
    The history of English begins a little after A.D. 600. The ancestors of the language were wandering in the forests of northern Europe. Their language was a part of Germanic branch of Indo-European Family. 英式英语和美式英语在历史起源和语言学上的差异(2):http://www.youerw.com/yingyu/lunwen_39236.html