It is the subject that TEFL teachers teach. There are many varieties of English, including, but not limited to, American English (AmE), Australian English (AuE), British English (BrE), Indian English, South African English, and so on. This wide-ranging reality has led to most specialists now preferring to use the term the English languages or Englishes.
Number of speakers of English
For various reasons it is difficult to be exact about the total number of native speakers of English but estimates vary from three hundred and nine million to three hundred and forty one million. This would rank English fourth in number of native speakers after Mandarin Chinese, Hindi/Urdu and Spanish.
On the other hand, if one were to attempt to include the number of individuals who speak English as a second language then the number becomes something in the order of one thousand five hundred million people - a larger number than that of any other language. Additionally English is used in international trade and industry to a greater extent than other languages.
History of English
See main article History of the English languages
Notwithstanding its many varieties, English has a long and varied history which is, not unnaturally, bound up with the history of Britain, the British Isles and its peoples.
Modern English is the product of various Germanic invasions, the Norman conquest, the British Empire and much else.
The structure of English
See main article Number of words in English
As we saw above, by the end of the Old English (Anglo-Saxon) period the size of the lexicon was around 50,000 different words. By the end of the Middle English period (1100-1500), that figure had doubled and during the Early Modern English period (1500-1700) it doubled yet again to 200,000 lexemes. And just for the record, partly as a result of the Industrial Revolution which started in the late 18th century, and twentieth-century global expansion, it would double once more to the approximately 400,000 lexemes of Modern English (1700 to the present).