These are two spellings of one word with identical meaning and pronunciation: "a charitable organisation" and "a charitable organization" say exactly the same thing. Organization is the only standard spelling in American English; in British, Irish and Australian English organisation is far commoner, though organization is also correct and is house style at Oxford University Press. Pick one and use it consistently.
The core difference: geography, not meaning
There is no difference in sense, register or pronunciation. This is a spelling variant, like colour and color, and it belongs to a much larger family: realise/realize, recognise/recognize, apologise/apologize, and the nouns built on them.
Which to use where
- United States and Canada: organization. The -ise form will be marked as an error.
- United Kingdom, Ireland, Australia, New Zealand, South Africa: organisation is the majority spelling and the safe default. Most British newspapers, the BBC and the Guardian style guide all use -ise.
- Academic and scientific publishing: check the style guide. Oxford University Press, Nature and the World Health Organization all use -ize, and this is not an Americanism — the -ize spelling is older in English than -ise and has been continuously used in Britain.
The rule you can apply
Match the rest of your document. If you have written colour, centre and realise, write organisation. If you have written color and center, write organization. Inconsistency within one piece of writing is the only thing a reader will genuinely notice as wrong. Word processors will enforce this for you if you set the document language to English (UK) or English (US).
The trap: not every -ise word has a -ize twin
This is where careful writers slip. A set of verbs end in -ise in all varieties of English, because the ending is not the Greek-derived suffix at all. Writing them with a z is simply wrong everywhere:
- advise, devise, revise, supervise, televise
- surprise, comprise, promise, exercise, compromise
- advertise, franchise, disguise, improvise
Running in the other direction, capsize, prize (the award) and size take z everywhere.
Also note that the -yse verbs do not follow the -ize rule. Even publishers who use organization spell it analyse, paralyse and catalyse in British English; analyze is American only.
Related forms
Whatever you choose applies right across the word family: organise/organize, organised/organized, organiser/organizer, disorganisation/disorganization. And when a name is involved, spell it as its owner does — the World Health Organization and the International Organization for Standardization both take z, even in British writing.
Frequently Asked Questions
Is 'organization' wrong in British English?
No. It is less common than organisation but it is fully correct, and it is the house style of Oxford University Press and several British scientific journals. What would be wrong is mixing the two spellings in one document.
Should I write 'World Health Organisation'?
No — write World Health Organization, with a z. Proper names keep the spelling their owners use, regardless of which side of the Atlantic you are writing on. The same applies to the International Organization for Standardization.