The root is the core of a stem, and the stem is the core of a word form. For instance, /luːz/ is the root of /luːzɚ/, and the latter is the stem of the plural form /luːzɚz/ (loosers). The definitions are as follows:

A stem is the lexical component of a word form which remains after discounting any inflection. With the exception of reliant stems (s. below), the stem represents a lexeme.

A root is a monomorphemic stem.

Distinguishing, in the lexicon and/or morphemicon of a language, roots and stems is basic for a number of further linguistic tasks:

1. The phonotactics of a language is relative to linguistic units of a certain level. One of these is the root. Thus, the phonotactic constraints on English roots will be different depending on whether depend is a root or is derived from a root pend.
2. Certain morphological processes, like German apophony, only work on roots.

The problems in distinguishing root and stem are manifold. One of them is the determination of morphological boundaries. In many cases, it is not immediately clear whether or not a given stem consists of more than one morpheme, i.e., concretely, whether a certain part of its significans is a derivational morpheme and another is a root. These problems are discussed on the page of segmentation problems.