The punctuation mark that comes at the end of a sentence; apparently coined by Shakespeare in “The Merchant of Venice.” The word Americans are familiar with, period, dates from the 16th century, but OED labels it “Now chiefly N. Amer.”; judging from the citations, that has been the case for about two hundred years. The mark has also sometimes been referred to as a point. (All of these probably eventually be supplanted by a new term: the dot indicating a period in computer addresses.)
Americans have taken to using full stop not to literally mean a period, but to emphasize that they are referring to a complete sentence, or by extension, a complete idea or phenomenon.
notoneoffbritishisms.com 
I just used full stop in a sentence of mine not too long ago. I think it’s perfectly acceptable. I use it the American way though, for emphasis.
I fail to see how this is a ‘Britishism’, as the period is surely an Americanism of full stop?
exactamundo!
Although the Puritans had probably never been exposed to Shakespeare (being religious extremists), they did bring to America the Elizabethan informal freedom with language. Later generations in Britain sought to formalize English, but the carefree make-it-up-as-you-go attitude continued in America. Thus many of the linguistic differences we see date to this period in time.
Perhaps confounding this is the use of “stop” at the ends of sentences or phrases in telegrams, popularized in movies esp. from the ’30s and ’40s.
Whats interesting is that sometimes I’ll say “that was the greatest thing ever, period” at the end of sentence as in the american sense, something I’ve picked up from reading internet forums and blogs most of which are american in origin. But the punctuation mark itself always will be a “full stop” to me.
Whether “full stop” or “period”, it’s a PUNCTUATION MARK. Useful in writing, certainly, but inserting it into speech for emphasis is at least daft, and often dodgy. It appears to be an attempt to close down any challenge or dispute by presenting something as incontestable. Anyone doing that protesteth too much, I think, and my suspicious mind immediately inclines to the view that the preceding statement is either untrue or dubious. Certainly, the speaker does not have confidence in it, otherwise (s)he would not resort to such a lead balloon of a rhetorical device.
The insertion of “-fact!” or, more commonly “-FACT!” at the end of a sentence, usually in writing, has a similar effect. It erects over the ordure a big red banner bearing the legends “I just made it up”, “I read it on the internet so it must be true” and “a bloke down the pub told me”. Whichever is the case, it makes little difference.
These are all “Help! I’m losing this argument, but my pride is at stake” constructions. The ruthless protagonist smells blood at that point and goes in for the kill. A kindlier one will just walk away, shaking the head and muttering “poor fool”.
In BrE ‘full stop’ is simply the name for the punctuation mark with no metaphorical use.
BrE has some Americanisms in clichéd phrases which contain metaphors that are seldom recognised as such. ‘In the dog house’ is one. As a phrase its meaning is wholly clichéd, with no realisation that it is metaphorical, meaning ‘in the kennel’, which is the word a Brit would use in every other instance. I suspect that in BrE emphatic ‘period’ is one such Americanism, in which the one-word phrase is understood to have a particular meaning that does not depend on understanding the original reference. I have not heard, nor do I expect to hear, a Brit ‘translate’ it as ‘full stop’ with the same meaning, for this cliché-based reason. If it does ever break into BrE it will I suspect be as Britishism that has boomeranged as an Americanism.
I’m intrigued by the undercurrent of hostility coming through in these posts. Surely usage dictates what is correct in any geographical location, be it North America, Britain, Australia, New Zealand, South Africa, India, or any other jurisdiction where English is widely spoken. After all, the similarities in most forms of English (which make them mutually intelligible), far outway the differences (which add character).
No metaphorical use? What about these examples from David Cameron?
“It is absolutely clear that Raoul Moat was a callous murderer, full stop, end of story. I cannot understand any wave, however small, of public sympathy for this man.” —public statement, 2010
“I’ve got no time for those who say this is a culturally relative issue – it is wrong, full stop, and we’ve got to stamp it out.” —speech on immigration, 2011
“And how can it be that after the tragic events at Charlie Hebdo in Paris, weeks were spent discussing the limits of free speech and satire, rather than whether terrorists should be executing people full stop?” —speech on extremism, 2015
It’s a little harder to search for examples in British media since they’re mixed in with literal uses, but they’re definitely there.
I had an employer once, who when particularly agitated, was given to saying, “Period. End of sentence. End of paragraph. End of statement.”