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In linguistics a lemma (plural lemmas or lemmata) is either of two things:
- Morphology, lexicography: the canonical form or citation form of a set of forms (headword); e.g., in English, run, runs, ran and running are forms of the same lexeme, with run as the lemma.
- Psycholinguistics: abstract conceptual form that has been mentally selected for utterance in the early stages of speech production, but before any sounds are attached to it.
A lemma in morphology is the canonical form of a lexeme. Lexeme, in this context, refers to the set of all the forms that have the same meaning, and lemma refers to the particular form that is chosen by convention to represent the lexeme. In lexicography, this unit is usually also the citation form or headword by which it is indexed. Lemmas have special significance in highly inflected languages such as Czech. The process of determining the lemma for a given word is called lemmatisation.
The psycholinguistics interpretation refers to one of the more widely accepted psycholinguistic models of speech production, referring to an early stage in the mental preparation for an utterance. Here, lemma is the abstract form of a word that arises after the word has been selected mentally, but before any information has been accessed about the sounds in it (and thus before the word can be pronounced). It therefore contains information concerning only meaning and the relation of this word to others in the sentence. This notion of lemma is similar to the Sanskrit sphota (6th c.), an invariant mental word, of which the sound is a feature.
Morphology and lexicography
In a dictionary, the lemma "go" represents the inflected forms "go", "goes", "going", "went", and "gone". The relationship between an inflected form and its lemma is usually denoted by an angle bracket, e.g., "went" < "go". The disadvantage of such simplifications is, of course, the inability to look up a declined or conjugated form of the word, although some dictionaries, like Webster's, will list "went". Multilingual dictionaries vary in how they deal with this issue: the Langenscheidt dictionary of German does not list ging (< gehen); the Cassell does.
The form that is chosen to be the lemma is usually the least marked form, though there are occasional exceptions; e.g., Finnish dictionaries list verbs not under the verb root, but under the first infinitive marked with -(t)a, -(t)ä.
Lemmas in different languages
In English, the citation form of a noun is the singular: e.g., mouse rather than mice. For multi-word lexemes which contain possessive adjectives or reflexive pronouns, the citation form uses a form of the indefinite pronoun one: e.g., do one's best, perjure oneself. In languages with grammatical gender, the citation form of regular adjectives and nouns is usually the masculine singular. If the language additionally has cases, the citation form is often the masculine singular nominative.
In many languages, the citation form of a verb is the infinitive: French aller, German gehen. In English it usually is the full infinitive (to go); the present tense is used for some defective verbs (shall, can; and must has only the one form). In Latin, Ancient Greek, and Modern Greek (which has no infinitive), however, the first person singular present tense is normally used, though occasionally the infinitive may also be seen. (For contracted verbs in Greek, an uncontracted first person singular present tense is used to reveal the contract vowel, e.g. φιλέω philéō for φιλῶ philō "I love" [implying affection]; αγαπάω agapáō for αγαπῶ agapō "I love" [implying regard]). In Japanese, the non-past (present and future) tense is used.
In Arabic, which has no infinitives, the third person singular masculine of the past tense is the least-marked form, and is used for entries in modern dictionaries. In older dictionaries, which are still commonly used today, the triliteral of the word, either a verb or a noun, is used. Hebrew often uses the 3rd person masculine qal perfect, e.g., ברא bara' create, כפר kaphar deny. For Korean, -da is attached to the stem.
Some phrases are cited in a sort of lemma, e.g., Carthago delenda est (literally, "Carthage must be destroyed") is a common way of citing Cato, although what he said was more like, Ceterum censeo Carthaginem esse delendam ("As to the rest, I hold that Carthage must be destroyed").
Difference between stem and lemma
A stem is the part of the word that never changes even when morphologically inflected, whilst a lemma is the base form of the verb. For example, from "produced", the lemma is "produce", but the stem is "produc-." This is because there are words such as production. 
Some lexemes have several stems but one lemma. For instance "to go" (the lemma) has the stems "go" and "wen-". (The "-t" of "went" may be considered as being derived from the past tense "-ed".)
When we produce a word, we are essentially turning our thoughts into sounds (a process known as lexicalisation). In many psycholinguistic models this is considered to be at least a two-stage process. The lemma is thus intermediate between the semantic level (where meaning is specified) and the phonological level (where the sounds of the word are specified). It is an abstract form containing syntactic information (about how the word can be used in a sentence), but no information about the pronunciation of the word. In this context, the lexeme is the phonologically specified form that is selected after the lemma.
This two-staged model is the most widely supported theory of speech production in psycholinguistics, although it has been recently challenged. For example, there is some evidence to indicate that the grammatical gender of a noun is retrieved from the word's phonological form (the lexeme) rather than from the lemma. This is easily explained by Caramazza's Independent Network model, which does not assume a distinct level between the semantic and the phonological stages (so there is no lemma representation); in this model, syntactic information about the word in this model is activated in the semantic or phonological level (so gender would be activated in the latter).
- Corpus linguistics
- Principal parts
- Root (linguistics)
- Null morpheme
- Uninflected word
- lexical markup framework
- ^ http://nltk.sourceforge.net/index.php/Book
- ^ Harley, T. (2005) The Psychology of Language. Hove; New York: Psychology Press: 359
- ^ e.g. Caramazza, A. (1997) How many levels of processing are there in lexical access? Cognitive Neuropsychology, 14, 177-208.
- ^ e.g., Starreveld, P. A. and La Heij, W. (2004) Phonological facilitation of grammatical gender retrieval. Language and Cognitive Processes, 19 (6), 677-711.
- ^ Caramazza (1997)