Working definition:

~ Through a single entry point simultaneously searching multiple resources (library catalogs, databases, web sites) from disparate vendors, platforms, protocols, and sources, and delivering an integrated set of results.

Also known as:

~ Broadcast search, metasearch, parallel search, search portal, cross-database search ...

Federated search and/or library portal?:

~ Search tool vs. resource discovery

For examples and vendors, see:

~ Library of Congress' Portals Applications Issues Group