A root (or root word) is a part of a word with lexical meaning that cannot be broken down further.[1] The term root originates in Hebrew linguistics and was unknown in Western linguistics until the 16th century, when Hebrew linguistics was discovered by non-Jewish linguists. The root word is the primary lexical unit of a word, and of a word family (this root is then called the base word), which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents. Content words in nearly all languages contain, and may consist only of, root morphemes. In some cases the term "root" is also used to describe the word without its inflectional endings, but instead with its lexical endings in place. For example, "chatters" has the inflectional root or lemma "chatter", but the lexical root chat. Inflectional roots are normally called stems, and a root in the stricter sense may be stated as a monomorphemic stem.

The traditional definition allows roots to be either free morphemes or bound morphemes. Root morphemes are essential for affixation and compounds. However, in polysynthetic languages with very high levels of inflectional morphology, the term "root" is generally synonymous with "free morpheme". Many such languages have a very restricted number of morphemes that can stand alone as a word: Yup'ik, for instance, has no more than two thousand.