What is a definition of the prefix?

Answer 1

A prefix is a word part added to the beginning of a word to create a word with a different meaning.

Examples : refill , undo, incomplete, precooked

Answer 2

a word or letters placed in front of word to change the meaning of the word.
EXAMPLE: the word "happy" means your feeling pleasure or content
But if you add "un" to the beginning of happy it becomes "unhappy" which is the opposite of being happy.