A lexeme-stem is a root, or a compound, plus possibly derivational affixes, that can combine with inflectional affixes if the language has any but does not contain any inflectional affixes.
This definition is from Haspelmath (2024: 68), where it is pointed out that "lexeme" (an abstract notion denoting the a set of related forms) must be defined in terms of the concrete notion of "lexeme-stem" (a kind of form). ❡
Note that this is not a very natural notion (similar to the "word" notion), and that it may not be helpful to use it as the foundation of an architectural distinction between inflection and word formation.