I've personally read a book or an article by Bertrand Russell where he defines philosophy. The definition --- IIRC --- is that philosophy is the study of the unknown. If it becomes a matter of fact, then it's called science.
Question. What is the publication where he mentioned that? (If you could, chapter, page number, edition would be great.)
I tried his autobiography. It could be there, but keywords such as philosophy, unknown, fact and obviously everywhere there. I've looked at his article on mathematical philosophy. It's likely not there. My best bet now is in The Scientific Outlook. It might very well be there, but I also can't find it.