In library classification systems, realia are three-dimensional objects from real life such as coins, tools, and textiles, that do not fit into the traditional categories of library material. They can be either human-made (artifacts, tools, utensils, etc.) or naturally occurring (specimens, samples, etc.), usually borrowed, purchased, or received as donation by a teacher, library, or museum for use in classroom instruction or in exhibits.

Archival and manuscript collections often receive items of memorabilia such as badges, emblems, insignias, jewelry, leather goods, needlework, etc., in connection with gifts of personal papers. Most government or institutional archives reject gifts of non-documentary objects, unless they have a documentary value. When accepting large bequests of mixed objects, they normally have the donors sign legal documents giving permission to the archive to destroy, exchange, sell, or dispose in any way those objects which, according to the best judgement of the archivist, are not manuscripts (which can include typescripts or printouts) or are not immediately useful for understanding the manuscripts.

Recently, the usage of this term has been criticized by librarians based on the usage of the term realia to refer to artistic and historical artifacts and objects. The noun phrase "real world object" has been suggested as a better term for describing the broader categories of three-dimensional objects in libraries.

Most libraries usually have a well written, legally tight acquisitions policy that rejects beforehand any object which is not some kind of print or text-based document. There are some exceptions. Children's libraries sometimes have a toy collection, whose individual items are lent out after being cataloged as realia, or under a more specific material designation such as toy, or game. Some large libraries have a special mandate to keep objects related to a literary collection.

Some very large libraries have a public relations department, which can find museum objects useful for enhancing or promoting the general collection. Such a library is more likely to prize realia for their associations with writers, subjects, or themes in the library's collections, rather than for their own intrinsic worth, artistic merit, historical significance, or scientific value. Examples might include a feather pen believed to have been owned by John Hancock; lead type from Benjamin Franklin's printing press; or a collection of Vietnam War era canteens, mess kits, uniforms, combat boots, etc. used in a "hands on" exhibit for children to illustrate the war.

Within the restricted domain of cataloging rules in the field of library and information science, the term "realia" is used to describe those mass-produced objects that incorporate documents or significant amounts of text (such as world globes, decks of quiz cards, and board games), but which have a format which makes it hard to incorporate them in the general collection or to describe them easily in the catalog. Special cataloguing rules are available to describe these objects.

Objects of realia, due to their diverse and compound nature, pose unique preservation challenges for libraries and archives. Unlike books and other traditional library materials, the artifactual value of these materials is key. In fact, when such items are unaccompanied by written documentation, as is often the case, the intellectual value sought by most library collections is often uncertain.

“We have a lot of hair,” Saundra Taylor of the Lilly Library told The New York Times, explaining that realia such as locks of hair, toys, and inkwells are often the unsolicited accompaniment to prized acquisitions of personal papers or book collections. Some libraries prize their realia, actively preserving and exhibiting it, while others simply keep it out of light and hope for the best.