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Definition
Discovery tool is a general, centralized index composed of data from publishers, content aggregators, and local sources (catalogs, institutional repositories, digitized collections, etc.), aimed at facilitating identification and access to documentary resources provided by the institution in question. Unlike federated search, which queries several publishers’ databases simultaneously via “connectors,” a discovery tool queries a unified database.
https://en.wikipedia.org/wiki/Discovery_system_(bibliographic_search)