Prefix: A word part added to the beginning of a word to change its meaning.
Examples of prefixes:
* un- (undo, unhappy)
* pre- (pregame, preview)
* re- (re-read, redo)
* mis- (misunderstand, misbehave)
* anti- (antibiotic, anti-social)
Examples of things that are *not* prefixes:
* Suffixes: These are word parts added to the end of a word to change its meaning (e.g., -ing, -ly, -ment)
* Root words: The base word that prefixes and suffixes are added to (e.g., "happy" in "unhappy")
* Standalone words: Words that have their own meaning and aren't parts of other words (e.g., "the", "cat", "run")
* Numbers: While some numbers can act as prefixes in specific contexts, they are not prefixes in general (e.g., "monopoly" uses "mono" meaning one)
In essence, anything that doesn't fit the definition of a prefix, which is a word part added to the beginning of a word to change its meaning, is not a prefix.