Real Dictionary

prefix
countable noun: A prefix is a letter or group of letters, for example 'un-' or 'multi-', which is added to the beginning of a word in order to form a different word. For example, the prefix 'un-' is added to ' happy' to form 'unhappy'. Compare affix and , suffix.
countable noun: A prefix is one or more numbers or letters added to the beginning of a code number to indicate, for example, what area something belongs to.

prefix in American English
a syllable, group of syllables, or word joined to the beginning of another word or a base to alter its meaning or create a new word (Ex.: pre- in precool, un- in unsure)
to fix to the beginning of a word, etc.; esp., to add as a prefix
noun: an affix placed before a base or another prefix, as un- in unkind, un- and re- in unrewarding
transitive verb: to fix or put before or in front

prefix in British English
noun: an affix that precedes the stem to which it is attached, as for example un- in unhappy
verb: to put or place before