The root word is organize not organise, I'm glad you use a dictionary but surely you can spell at a 2nd grade level
You must be in second grade... or you probably didn't read that wikipedia article at all.
Different countries = (in this case) slightly different languages = different spellings.
Just to save you the trouble
-ise / -ize
American spelling accepts only -ize endings in most cases, such as organize, recognize, and realize. British usage accepts both -ize and the more French-looking -ise (organise, recognise, realise). However, the -ize spelling is now rarely used in the UK in the mass media and newspapers, and is hence often incorrectly regarded as an Americanism
Anyway, let's discontinue the argument, people spell stuff different. Case solved.