Why do 'organization' and 'organisation' both seem to be commonly accepted spellings, and when is one used over the other?

Question

Looking at several online dictionary resources, it seems that the accepted spelling of "organization" is with a 'z', however, even on this site, "organisation" is frequently used as the spelling. Why the difference, and when is it appropriate to use one over the other?

EDIT: Here is a link to the Oxford Dictionary article, just for ease of reference: http://www.oxforddictionaries.com/us/definition/american_english/organization?q=organisation

The link for the non-American-English article as well: http://www.oxforddictionaries.com/definition/english/organization

Answer 1

British vs American spelling?

It's not that simple.

I'm British, despite the Polish surname (father was one of the Polish RAF pilots in WWII), and when I was at school in the 1950s and 60s it was "organization"; which is still the preferred spelling in the Oxford English Dictionary, - despite the Media spelling it with an 's' instead of a 'z'. What you may not be aware of, being on the other side of the 'pond', is that Oxford is fighting something of a 'rear guard action' against the ever encroaching 's' and the disappearance of the 'z'. Being a Northern England inhabitant I also find the stronger 'z' more sensible from a pronunciation point of view than the softer 's' of the South.

I remember that it was not until the mid 1970s that some people started to tell me that I was using the American spelling. In fact most British people that I meet, including my students here at the university where I teach, believe this to be true. Hell, the whole continent of Australia believe the 'z' is the American spelling!

Well at nearly seventy, I shall continue to spell all such -ize words with a 'z', if only because I see young people not even attempting to pronounce my name because it has a 'z' in it! :-)

Answer 2

It's an American and British English spelling difference:

The history of its spelling differences starts in the early 18th century, English spelling was not standardized. Differences became noticeable after the publishing of influential dictionaries. Current British English spellings follow, for the most part, those of Samuel Johnson's Dictionary of the English Language (1755), whereas many American English spellings follow Noah Webster's An American Dictionary of the English Language of 1828. Webster was a strong proponent of spelling reform for reasons both philological and nationalistic. Many spelling changes proposed in the US by Webster himself, and in the early 20th century by the Simplified Spelling Board, never caught on. Among the advocates of spelling reform in England, the influences of those who preferred the Norman (or Anglo-French) spellings of certain words proved decisive. Subsequent spelling adjustments in the UK had little effect on present-day US spelling, and vice versa. While in many cases American English deviated in the 19th century from mainstream British spelling, on the other hand it has also often retained older forms.

As Ngram shows, the AmE spelling is organization. The link suggested by Andrew Leach gives other important information on spelling differences between BrE and AmE related to your question.

Source: https://in.answers.yahoo.com