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.
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?
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.