A collection of one or more PL/SQL program units that are stored together in a file on the client file system or in the database. Program units stored in a library can be referenced by several applications at once. Unlike stored program units, which reside and execute in the database, program units in libraries always load and execute within the PL/SQL engine on the client side. To reference program units that are stored in a library, you must first attach the library. See also: attached library.