The term "orphan work" is used to describe a situation where it is difficult or impossible to contact the copyright holder of a copyrighted work. This situation can arise for many reasons. The author could have never been publicly known because the work was published anonymously, or the work may never have been published at all. Even if the author is known, the copyright may have been transferred to a relative, estate, or publisher. Nearly any work, where a resonable effort has been made to contact the copyright owner fails, can be considered an orphaned.
Should you use an orphaned work? Quite simply, you should balance your need against the potential risk of being sued by an unidentified rights holder. In addition, before using an orphaned work, you might consider these alternatives:
Consider the "potential market" for the work in question, and the possible harm to that market caused by your use of the work. If you discover that there is no way for you to obtain the proper permissions for use of the work, you should reevaluate the fourth factor in the fair use analysis. You may find that this factor will weigh in favor of fair use.
Can you locate an alternative source with an identifiable copyright holder? If so, contact that copyright holder to obtain permissions for use of the work. You may also be able to locate a similar resource that is within the public domain.
Rethink Your Intended Use
Rather than use an entire work, can you cite a small portion of the work, or even paraphrase relevant points? Relying on quotations, paraphrasing, and proper citation methods may be able to help you avoid the copyright permissions process altogether.