In linguistics, a word stem is a part of a word responsible for its lexical meaning. Typically, a stem remains unmodified during inflection with few exceptions due to apophony (for example in Polish, miast-o ("city") and w mieść-e ("in the city"); in English, sing, sang, and sung, where it can be modified according to morphological rules or peculiarities, such as sandhi)
Uncovering and analyzing cognation between word stems and roots within and across languages has allowed comparative philology and comparative linguistics to determine the history of languages and language families. The term is used with slightly different meanings depending on the morphology of the language in 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.
Root vs stem
By attaching the morpheme -ship to the root word friend (which some linguists call a stem, too), the new word friendship was synthesized. While an s can be attached to friendship to form friendships, it can not be attached to the root within it to form friendsship. A stem is a base from which all its inflected variants are formed. For example, the stabil- (a variant of stable unable to stand alone) is the root of the destabilized, while the stem consists of de·stabil·ize, including de- and -ize. The -(e)d, on the other hand, is not part of the stem.
Stem may either consist of a root (e.g. run) alone or a compound word, such as meatball and bottleneck (examples of compound nouns) or blacken and standardize (examples of compound verbs). The stem of the verb to wait is wait: it is the part that is common to all its inflected variants.
- wait (infinitive, imperative, present subjunctive, and present indicative except in the 3rd-person singular)
- waits (3rd person singular simple present indicative)
- waited (simple past)
- waited (past participle)
- waiting (present participle)
Citation forms and bound morphemes
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, word 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). However, the equivalent Spanish verb stem corr- never appears as such because it is cited with the infinitive inflection (correr) and always appears in actual speech as a non-finite (infinitive or participle) or conjugated form. Such morphemes that cannot occur on their own in this way are usually referred to as bound morphemes.
In computational linguistics, the term "stem" is used for the part of the word that never changes, even morphologically, when inflected, and a lemma is the base form of the word. For example, given the word "produced", its lemma (linguistics) is "produce", but the stem is "produc-" because of the inflected form "producing".
Paradigms and suppletion
A list of all the inflected forms of a word stem is called its inflectional paradigm. The paradigm of the adjective tall is given below, and the stem of this adjective is tall.
- tall (positive); taller (comparative); tallest (superlative)
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-.
- good (positive); better (comparative); best (superlative)
Oblique stem 
Both in Latin and Greek, the declension (inflection) of some nouns uses a different stem in the oblique cases than in the nominative and vocative singular cases. Such words belong to, respectively, the so-called third declension of the Latin grammar and the so-called third declension of the Ancient Greek grammar. For example, the genitive singular is formed by adding -is (Latin) or -ος (Greek) to the oblique stem, and the genitive singular is conventionally listed in Greek and Latin dictionaries to illustrate the oblique.
Historically, the difference in stems arose due to sound changes in the nominative. In the Latin third declension, for example, the nominative singular suffix -s is combined with a stem-final consonant. If that consonant was c, the result was x (a mere orthographic change), while if it was g, the -s caused it to devoice, again resulting in x. If the stem-final consonant was another alveolar consonant (t, d, r), it elided before the -s. In a later era, n before the nominative ending was also lost, producing pairs like atlas, atlant- (for English Atlas, Atlantic).
- Lemma (morphology)
- Morphological typology
- Morphology (linguistics)
- Principal parts
- Root (linguistics)
- Stemming algorithms (computer science)
- Thematic vowel
- Houghton Mifflin Harcourt, The American Heritage Dictionary of the English Language: Indo-European Roots Appendix, Houghton Mifflin Harcourt.
- Geoffrey Sampson; Paul Martin Postal (2005). The 'language instinct' debate. Continuum International Publishing Group. p. 124. ISBN 978-0-8264-7385-1. Retrieved 2009-07-21.
- Paul Kroeger (2005). Analyzing grammar. Cambridge University Press. p. 248. ISBN 978-0-521-81622-9. Retrieved 2009-07-21.
- What is a stem? – SIL International, Glossary of Linguistic Terms.
- Bauer, Laurie (2003) Introducing Linguistic Morphology. Georgetown University Press; 2nd edition.
- Williams, Edwin and Anna-Maria DiScullio (1987) On the definition of a word. Cambridge MA, MIT Press.