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, i.e. atomic (unanalyzable) lexical morphemes, but a stem can also 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 [[photo-graph]-er] is photographer and its only other inflected form is the plural photographers.
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.
In languages with very little inflection, such as English and Chinese, the stem is usually not distinct from the "normal" form of the word (the lemma, citation or dictionary form). However, in other languages, stems may rarely or never occur on their own. For example, the English verb stem run is indistinguishable from its present tense form (except in the third person singular); but the equivalent Spanish verb stem corr- never appears as such, since it is cited with the infinitive inflection (correr) and always appears in actual speech as a non-finite (infinitive or participle) or conjugated form. Morphemes like Spanish corr- which can't occur on their own in this way, are usually referred to as bound morphemes.
Some paradigms do not make use of the same stem throughout; this phenomenon is called suppletion. An example of a suppletive paradigm is the paradigm for the adjective good: its stem changes from good to the bound morpheme bet-.
Research Reports from University of Amsterdam, Department of Psychonomics Provide New Insights into Ageing, Neuropsychology and Cognition.(Report)
Jan 17, 2011; A new study, 'Priming effects from young-old to very old age on a word-stem completion task: minimizing explicit contamination,'...