Glossary entry

A published work in which, under the provisions of the Code, or by a ruling of the Commission, names or nomenclatural acts cannot be established. Such works include those which
(1) were issued before 1758 [Art. 3], or
(2) do not consistently apply the Principle of Binominal Nomenclature [Art. 11.4], or
(3) are published anonymously after 1950 [Art. 14], or
(4) carry a disclaimer, or
(5) the Commission has ruled to be unavailable. For the use of information affecting nomenclature in unavailable works, see Articles 12.2.1, 12.2.7 and 13.1.2.

Source:
International Commission on Zoological Nomenclature (1999) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.

See also:
disclaimer
published work
Principle of Binominal Nomenclature