A keyword describing the subject matter of a document, which when occurring as part of an alphabetical index enables documents relating to a particular topic or combination of topics to be retrieved. Frequently attributive, especially in "Uniterm system": an indexing system employing such keywords.
The Uniterm system of indexing was invented by Mortimer Taube (1910–65).
1950s. From unit + term, in some forms as a blend of the two words.