An affix can be added to the root of a word to change its meaning. An affix added to the front of a word is known as a prefix. One added to added to the back is known as a suffix. (Sometimes, prefixes are hyphenated.)
incapable (prefix "in"), ex-President (prefix "ex"), laughing (suffix "ing")
When connected to a title, a prefix is not written with a capital letter (unless it starts the sentence).
I met ex-President Clinton.
Glossary of grammatical terms