A root word is a word with no prefixes or suffixes, meaning that it is the primary lexical unit of a set of words and represents the principle semantic meaning of the set. The root word is morphologically simple, does not have any inflectional endings, and cannot be further broken down into meaningful parts. A root word will be common to various derived and inflectional forms of the word. Some root words can be used by themselves, while others are only meaningful when combined with various affixes.

In natural language processing, the root of a word is sometimes referred to as the stem, although in linguistics the stem has a somewhat different meaning, which is the part of a word common to all inflected variants.