A prefix is added to the front of a word to change its meaning. Both prefixes and suffixes (added to the back) are affixes. Prefixes are sometimes hyphenated.
anti-war (prefix "anti"), preconception (prefix "pre"), postnatal (prefix "post")
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