Collective collection has become part of the librarian argot. Coined by our colleague, Lorcan Dempsey, the term emerged from OCLC Research’s work analyzing library collections at scales above the institutional level—group, consortial, regional, national, and even global.
The best way of understanding collective collections is to start with WorldCat, which is a global registry of library holdings. Taken together, these holdings document the sum total of materials available in library collections worldwide—or at least a close approximation. In this sense, WorldCat represents the collective collection of the global library system as a whole.