It's a little bit confusing. But the word does exist and means the same as Ti.
Check http://en.wikipedia.org/wiki/Note#History_of_note_names for a description. And http://en.wikipedia.org/wiki/Solfege for a more elaborate description along with some interesting trivia on how these words came to be.
Si is kept in most romantic speaking countries, and then some more. I'm unsure why English speaking countries felt Si could be confused for So(l). But the noun is still existing notheless. Both because of the chromatic scale, and because my Webster's says so. :)