from The Grammarist:
Realise and realize are different spellings of the same word, and both are used to varying degrees throughout the English-speaking world. Realize is the preferred spelling in American and Canadian English, and realise is preferred outside North America. The spelling distinction extends to all derivatives of the verb, including realised/realized, realising/realizing, and realisation/realization.
Although realize is now regarded by many in the U.K. and Australasia as the American spelling, it is not an Americanism. In fact, the -ize spelling variant is older than -ise—realize predates the United States and Canada by nearly two centuries—and has been the preferred spelling throughout most of the word’s history in English. If we can believe the ngram below, which graphs the use of realize and realise in British books and journals published between 1800 and 2000, realise had a brief ascendancy in British English from the late 19th century through the early 20th, but realize was preferred before around 1875 and is again preferred today—perhaps because of the influence of dictionaries like Oxford, Cambridge, and Collins, which encourage -ize over -ise.