From The Art and Popular Culture Encyclopedia
In linguistics, a stem (sometimes also theme) is the part of a word that is common to all its inflected variants. Stems are often roots, e.g. atomic, its root is atom, but its stem is atom·ic. A stem can be morphologically complex, as seen with compound words (cf. the compound nouns meat ball or bottle opener) or words with derivational morphemes (cf. the derived verbs black-en or standard-ize). Thus, the stem of the complex English noun photographer is photo·graph·er, but not photo. For another example, the root of the English verb form destabilized is stabil-, a form of stable that does not occur alone; the stem is de·stabil·ize, which includes the derivational affixes de- and -ize, but not the inflectional past tense suffix -(e)d. That is, a stem is that part of a word that inflectional affixes attach to.
The exact use of the word 'stem' depends on the morphology of the language is question. In Athabaskan linguistics, for example, a verb stem is a root that cannot appear on its own, and that carries the tone of the word. Athabaskan verbs typically have two stems in this analysis, each preceded by prefixes.