An affix is a bound morpheme that attaches to a root word to form either a new word or a new form of the same word.
Note:
Morphemes (prefixes, suffixes and root words), are the smallest meaningful units of meaning.
Morphemes can be either single words (free) or parts of words (bound).
Free morphemes: can stand alone as its own word: dog, gentle, gem
Bound morphemes: can only occur as part of a word: dogs, climbed, unhappy, teacher