A lexeme is an abstract unit of morphological analysis in linguistics, that roughly corresponds to a set of words that are different forms of the same word. For example, English run, runs, ran and running are forms of the same lexeme. A related concept is the lemma (or citation form), which is a particular form of a lexeme that is chosen by convention to represent a canonical form of a lexeme. Lemmas are used in dictionaries as the headwords, and other forms of a lexeme are often listed later in the entry if they are unusual in some way.

A lexeme belongs to a particular syntactic category, has a particular 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 for run has a present third person singular form runs, a present non-third-singular form run, a past form ran, and a present participle running.

The use of the forms of a lexeme is governed by rules of grammar; in the case of English verbs such as run, these include subject-verb agreement and compound tense rules, which determine which form of a verb can be used in a given sentence. A lexicon consists of lexemes. In many formal theories of language, lexemes have subcategorized frames to account for the number and types of complements they occur within sentences and other syntactic structures.