Compare: Sovietism, Anti-Sovietism, Anti-Americanism
An Americanism is a word or phrase used commonly in America that has been more recently introduced into British English (for example), especially through the popular media of television and movies. However the term is also used for old usages which have persisted in America while falling out of favour in Britain and elsewhere - often in ignorance that this is what has happened.
Many expressions which spread from America are regarded as Americanisms for a while and then gradually become assimilated into British English. But if there is a direct equivalent this tends not to happen; the American flavour is retained. For instance, British people may occasionally use the phrase "step on the gas" but remain aware that it is an Americanism because they continue to fill their cars with "petrol" (and so the phrases "step on it" or "put your foot down" remain much more widely used).
Some writing conventions fall into the second category. Sometimes the expressions "Mr. Smith" and "July 15th, 1960" are regarded as Americanisms, with "Mr Smith" and "15th July, 1960" their British equivalents. But some English persons still living can remember being taught in school to write "Mr. Smith" rather than the newer usage "Mr Smith". More recently it has been taught that only abbreviations which lack their final letters should be given a full stop (period), resulting in "Col." and "Rev." but "Mr" and "Dr" as examples. Regarding dates, a look into a couple of British novels shows the former date style being used in 1859 while by 1927 the two were being used interchangeably (by the same character!). Today most British people would regard either form as acceptable. On the other hand, writing the date as 7/15/60 (rather than 15/7/60) would definitely be seen as American.
Similar remarks presumably apply to other non-American varieties of English.