A letter or group of letters added to the end of a word or root (i.e., a base form), serving to form a new word or functioning as an inflectional ending. Adjective: suffixal.
There are two primary types of suffix in English:
A derivational suffix (such as the addition of -ly to an adjective to form an adverb) indicates what type of word it is.
An inflectional suffix (such as the addition of -s to a noun to form a plural) tells something about the word's grammatical behavior.