A discovery system is a bibliographic search system based on search engine technology. It is part of the concept of Library 2.0 and is intended to supplement or even replace the existing OPAC catalogs. These systems emerged in the late 2000s in response to user desire for a more convenient search option similar to that of internet search engine. The results from searching a discovery system may include books and other print materials from the library's catalog, electronic resources such as e-journals or videos, and items stored in other libraries. Terminology The term discovery layer has been described as an overarching term that can include: , a graphical user interface that library users search or browse with. It includes search-engine like amenities such as spelling correction, tolerance for punctuation differences, and typeahead search. This idea is also called a next-generation catalog. , an interconnected search system, allowing library users to search not only the catalog of library print materials, but various digital resources and perhaps resources located in other libraries. The discovery system queries more than one data source upon a single user request; this is sometimes called federated search. The term indicates that the software is modular and can be used to emphasize the separability of catalog search and browse functionality from an integrated library system (ILS). The distinction between discovery , , and is not rigorous and the terms are sometimes used as synonyms. These "inconsistencies were in part due to the field's newness" when the terms were being created. History OPACs Users searching for print materials (such as books) at a library once used card catalogs, and later computerized catalogs called OPACs. Searching for resources other than material in the catalog, such as electronic resources, was (or is) done with separate tools. Using card or computerized catalogs well required skills and jargon particular to libraries. As computerization advanced, OPACs were integrated with other library systems, like acquisition and circulation systems. The resulting monolithic software systems were named integrated library systems. Changing expectations As the web became more widespread, library users developed "the expectation of being able to discover the collection in a search engine style". Gradually, discovery interfaces were created to be more forgiving of misspellings and punctuation choices than historical OPACs, and to offer features like suggestion of related search terms and faceted search. Reference librarians in the mid-2000s also spent "a lot of time talking