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.
By attaching the morphem -ship to the friend (a root which some linguists call stem, too), the new word friendship was synthesized. While an s can be attached to the friendship, it can not be attached to the root within it to form a plural. 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 of 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)
Lemma (morphology)
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).