http://languagearts.pppst.com/affixes.html
What are Prefixes and Suffixes?
Prefixes and suffixes are grammatical and lingual "affixes." Prefixes are affixed before and suffixes after a base word or word stem to add information. For example, with the word "prehistoric," the prefix is "pre-" meaning "before," the base word is "history" meaning "recorded events and knowledge", and the suffix is "-ic" meaning "relating to the science of."
In other words, "prefix" simply refers to an attachment before or in front of, in this case, a shorter word or stem. In lingual terms, a "stem" is the main part of a word to which prefixes and suffixes can be added and may not necessarily be a word itself, such as "dod" in "doddle"