Bagi,
British people call it Search Engine Optimisation and American people call it Search Engine Optimization so they are both correct depending on the country you come from

When I used to do SEO services, I usually write it as follows: Search Engine Optimisation and then put the American version in bracket (Search Engine Optimization ) so that search engine both British and American will be targeted.

This will be interesting as the British lanquage is changing all the time.
Words that are commonly used are now accepted into the Oxford Dictionary so after checking a few .co.uk sites and seeing the american version used what will happen???

John,
Just think of it as recyling, we gave the US their language, now they have made some modifictions to the language and sent it back to us and we are begining to use the recycled English language

Ah see...Eco freindly as ever

Well thank you for the answers they make this question clear for me. I currently live through the "why" period concerning the Internet.

Sad though it is, it should be SEm anyhow Search Engine Marketing (although none of us market search engines??? )