A word stem is the form of a word that can accept affixes (suffixes and prefixes) and is thus common to all inflected variants. For example, the stem of the words circle, circles, circular, and circling would be “circl”. The stem of a word may or may not be a word itself (as in the case of “circl”, which is not a word on its own.)
The concept of a word stem is closely related to the root word, although the two terms are not precisely synonymous.
In natural language processing, stemming is the process of reducing words to their stem forms.