A kept collection meant to be drawn on by others. Its purpose is not storage but retrieval.
Latin librarium (bookcase), from liber (book) — and liber originally meant the INNER BARK OF A TREE, the surface written on before papyrus reached Rome. Via Old French librairie. Most European languages took the Greek βιβλιοθήκη (bibliothēkē, book-repository) instead; English kept the Latin, which is why French librairie means bookshop rather than library.
The single source of truth for every code definition in the house. The bark-to-book path is the point: a library is a material that holds writing, and the law here is that a definition lives in exactly one place and is referenced everywhere else.