A prefix is an affix

Affix

An affix is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes...

 which is placed before the root of a word. Particularly in the study of languages,a prefix is also called a preformative, because it alters the form of the words to which it is affixed.

Examples of prefixes:

• unhappy : un is a negative or antonymic prefix.
• prefix, preview : pre is a prefix, with the sense of before
• redo, review. : re is a prefix meaning again.

The word prefix is itself made up of the stem fix (meaning attach, in this case), and the prefix pre- (meaning "before"), both of which are derived from Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 root

Root (linguistics)

The root word is the primary lexical unit of a word, and of a word family , which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents....

s.