A lexeme (/ˈlɛksiːm/ ^ⓘ) is a unit of lexical meaning that underlies a set of words that are related through inflection. It is a basic abstract unit of meaning,^[1] a unit of morphological analysis in linguistics that roughly corresponds to a set of forms taken by a single root word. For example, in the English language, run, runs, ran and running are forms of the same lexeme, which can be represented as RUN.^{[note 1]}

One form, the lemma (or citation form), is chosen by convention as the canonical form of a lexeme. The lemma is the form used in dictionaries as an entry's headword. Other forms of a lexeme are often listed later in the entry if they are uncommon or irregularly inflected.

The notion of the lexeme is central to morphology,^[2] the basis for defining other concepts in that field. For example, the difference between inflection and derivation can be stated in terms of lexemes:

• Inflectional rules relate a lexeme to its forms.
• Derivational rules relate a lexeme to another lexeme.

A lexeme belongs to a particular syntactic category, has a certain meaning (semantic value), and in inflecting languages, has a corresponding inflectional paradigm. That is, a lexeme in many languages will have many different forms. For example, the lexeme RUN has a present third person singular form runs, a present non-third-person singular form run (which also functions as the past participle and non-finite form), a past form ran, and a present participle running. (It does not include runner, runners, runnable etc.) The use of the forms of a lexeme is governed by rules of grammar. In the case of English verbs such as RUN, they include subject–verb agreement and compound tense rules, which determine the form of a verb that can be used in a given sentence.

In many formal theories of language, lexemes have subcategorization frames to account for the number and types of complements. They occur within sentences and other syntactic structures.

A language's lexemes are often composed of smaller units with individual meaning called morphemes, according to root morpheme + derivational morphemes + affix (not necessarily in that order), where:

• The root morpheme is the primary lexical unit of a word, which carries the most significant aspects of semantic content and cannot be reduced to smaller constituents.^[3]
• The derivational morphemes carry only derivational information.^[4]
• The affix is composed of all inflectional morphemes, and carries only inflectional information.^[5]

The compound root morpheme + derivational morphemes is often called the stem.^[6] The decomposition stem + desinence can then be used to study inflection.

• Ending (linguistics)
• Inflection
• Lemma
• Lexicon
• Lexical word vs. grammatical word
• Marker (linguistics)
• Multiword expression
• Null morpheme
• Root (linguistics)
• Stem
• Syntagma (linguistics)
• Word family

• The dictionary definition of lexeme at Wiktionary