initialised or initialized which one is correct spelling? [duplicate]

I have often seen initialised in lots of text, but when I want to write it in Microsoft office word, it says it was misspelled and it should be initialized instead of initialised.

so here is my question:

Which one of them is the correct spelling?

and if both of them are correct, when I should use one or the other? Is there any rule regarding this?

Additional note: when I was writing this post, it would give me a spelling error notice each time I wrote initialised.

The variant of initialize with an s is the British spelling; the one with a z is standard American.

See here.

If you’re British, you’re likely to use the s variant. If you’re an American (like me), you’ll use the z variant.

Incidentally, many words that replace a z with an s tend to be British spelling, like “realise.” [American variant=realize]