Is it Optimization or Optimisation?

I have been building websites for some time now and I have learned all about Search engine optimization (SEO). At the very beginning of learning about SEO, I kept running into articles that spoke about Search Engine Optimization but their spelling was a bit different. It was spelled Optimisation, with a S instead of a Z. I really started to frown upon this due to the fact that they weren't even spelling the word correctly. How can they attempt to teach others about SEO?

At first I didn't even think about checking into the spelling of the word to see which one was correct because the spelling seemed totally obvious to me. Optimization- [op-tuh-muh-zey-shuhn]. A couple years later, I started to doubt my own spelling and SEO knowledge so I researched the definitions of both spellings. I researched "what is optimization?" and found this definition: 1. The fact of Optimizing; making the best of anything. 2. "The condition of being optimized." I then searched for the word with a different spelling: Optimisation- Synonym for optimization. So basically, Optimisation is not necessarily incorrect. However, if you are typing the word in an article the spell checker automatically adds the red line underneath the word Optimisation and not Optimization. Why is this? Both of them mean the same, why does everyone think they are mispelled?

Research has shown that British English and American English some times spell the same word in two different ways. British English spells Optimisation with an S and American English would spell Optimization with a Z. Another example of spelling the same word differently would be the word color. British English spells it Colour and American English spells it Color. So the spelling of certain words depend on which part of the world you are from. Another example of this is the word recognize and recognise.

All comments and samples are welcome below.