For titles which are collected in microform, paper issues are retained until the microform is received. Descriptor - A word or phrase used as a subject heading. It is part of a controlled vocabulary used for a specific database. Compare with Subject Headings , Keyword Searching . Dictionary - A reference source that provides meanings of words and other information.
Specialized dictionaries are available for many subject areas.
Dissertation - Original research usually required for a Ph.D. degree. Document Delivery - A service which allows users to order copies of materials, usually journal articles, via an online communications link. The document may be delivered by mail, fax, email, or sent directly to the user's workstation. See also Interlibrary Loan . Encyclopedia - A reference source containing information on a variety of topics.