A prefix is an affix that comes before the word's stem. When you add it to the beginning of a word, it becomes a different term. When the prefix un- is added to the word happy, the word unhappy is created.
A prefix (also known as an affix) is a set of letters that is added to the beginning of a word. The meaning of a word is altered by prefixes. They can be used to make a word negative, show repetition, or express a point of view.
A prefix is a set of letters that come before a word's root. For example, the word "unhappy" is formed by combining the prefix "un-" [which meaning "not"] with the root (or stem) word "happy," resulting in the phrase "not happy."
The meaning of the basic word is frequently altered by a prefix. Consider the word "happy." When the prefix un is added to the word happy, it creates a new word with a different meaning: unhappy.