a cohesive collection of non-circulating research materials held together by provenance or by a thematic focus
an institution or an administrative unit of a library responsible for managing materials outside the general library collection, including rare books, archives, manuscripts, maps, oral history interviews, and ephemera
The term special collection is general enough it can refer to almost any collection of research materials. When referring to an institution or administrative unit, it is usually used in its plural form but construed as singular.
The most common use of the term is in the plural sense, where it usually refers to an institution or unit that maintains more than one special collection. (But the singular special collection can also refer to such an institution.) This use of the term is common in the titles of units in libraries, such as Special Collections Research Center or Special Collections and University Archives. Although special collections here is plural, it is plural only in the way that archives is plural in a North American context: One would say, for example, “Special collections holds an amazing collection of historic photographs.”
But a special collection is also nothing more than an individual collection of library materials of research value: a collection of books documenting binding techniques of the eighteenth century, a donated collection of movie posters from the 1930s and 1940s, or an assembled collection (archivists might say an artificial collection) of lantern slides documenting one region of the country. Special collections often, but not always, have artifactual value, and special collections in these times often consist of born-digital materials. Most special collections are donations or purchased acquisitions, but the archival records of the parent institution are treated as a special collection in some institutions.
Some special collections hold many of these different formats of materials, and others have only one or two. What sets them apart is the specialness of their resources, the stark difference they usually show compared to the paper volumes, online databases, and even the surviving microfilm of the general collection of a library.