The inventorium of a language is the set of forms and constructions that cannot be predicted from other forms or constructions.
This definition is from Haspelmath (2024: 79). Since Bloomfield (1933), the set of unpredictable elements has also been called "lexicon", but this term has also been used for an individual's "mental lexicon" (or mentalicon), or for the set of all lexemes (or lexemicon). But these are very different notions, because a mental lexicon may contain a lot more than the unpredictable elements, and it is not only lexemes that are unpredictable (many inflected forms are unpredictable, as well as many larger idiomatic expressions). Moreover, the term "lexicon" rarely includes the constructions of a language (though Jackendoff & Audring 2020 use "extended lexicon" in this way).