A prefix is an affix that is joined before a root or stem .
The prefix un- attaches to the front of the stem selfish to form the word unselfish.
Mish, Frederick (editor). 1991.Webster's ninth new collegiate dictionary. Springfield, MA: Merriam-Webster. 1,564 pages. 0877795088; indexed 0877795096; deluxe 087779510X.
Hartmann, R.R.K., and F.C. Stork. 1972.Dictionary of language and linguistics. London: Applied Science.
Crystal, David. 1980.A first dictionary of linguistics and phonetics. Boulder, CO: Westview.