Lexeme is a term that is used to express the idea that inflected forms, which are words themselves, are still variants of one single word.
if we take a look at the declension of the Russian adjective bol'soj 'large, big, grand', we find that this word has a variety of forms:
(i) bol's-oj nominative singular masculine bol's-omu dative singular masc. bol's-ix genitive plural masc. (ii) bol's-aja nom.sg. feminine bol's-oj dat.sg. fem. bol's-ix gen.pl. fem.
In a sense each of these variants is itself a word: the term 'word form' is used for them, and the term 'lexeme' is used for the more general sense, so that we can say that bol'somu is a form of the lexeme BOL'SOJ (lexemes are represented in upper case).
- Spencer, A. 1991. Morphological Theory, Blackwell, Oxford.