A fundamental unit of the lexicon of a language, i.e., the minimal lexical unit.
For a natural language, a lexeme is equivalent to a root word, that is, a base unit with no inflectional or derivational modification. For example, flood (the noun), flood (the verb), floods, flooded, and flooding are all forms of the lexeme flood.
For a programming language, lexemes are the keywords, literals, identifiers, operators, and punctuation. The lexical analysis phase of the compiler converts strings into a sequence of lexemes, which are then passed to the parser for syntactic analysis.