The original language, that is to say the language spoken by the people of its country of origin. As opposed to American English (aka 'Anglo-American') and worst of all Microsoft English (though there are a few others which vary from the original to a lesser extent).
The most notable change in the American variant of English is that it has hundreds of changes to spelling, mostly to make it more phonetic, though there are a few words which have been added or their definition changed.
Microsoft English is the language that Microsoft Word describes as 'English (UK)', and remains a mystery to us mortals. No-one knows what the f*ck it is, but it's certainly not quite the same as anything else. I really want some of whatever the programmers were smoking when they made Microsoft Word's spelling/grammar checker (emphasis on the grammar check).

when english people talk using mostly english slang.
near to imposible for people from other counties to understand.
sometimes it can be misunderstood by english people.