The U. S. Copyright Office defines it thus:
“Copyright law protects a work from the time it is created in a fixed form. From the moment it is set in a print or electronic manuscript, a sound recording, a computer software program, or other such concrete medium, the copyright becomes the property of the author who created it. Only the author or those deriving rights from the author can rightfully claim copyright.
There is, however, an exception to this principle: “works made for hire.”
If a work is made for hire, an employer is considered the author even if an employee actually created the work. The employer can be a firm, an organization, or an individual.
The concept of “work made for hire” can be complicated. This circular refers to its definition in copyright law and draws on the Supreme Court’s interpretation of it in Community for Creative Non-Violence v. Reid, decided in 1989.”
Their circular provides more detail.
Probably the most obvious example of “work made for hire” is work written by an employee as part of the scope of their employment. Think journalists. Other categories depend on an agreement between the parties. Thus, perhaps if you were employed by a publisher as a Production Director and wrote a few last-minute entries for an Encyclopedia, fleshing out its coverage of baseball, your work would only be work made for hire if you had a piece of paper in which your publisher asked you to do the work under these terms. The fact that I didn’t have such a piece of paper doesn’t really matter, as I had/have no intention of suing for what is an utterly worthless right. Of course the law courts might decide that this was in fact part of the scope of my employment even though my job didn’t involve writing stuff, and although I wrote in the evenings while not in the office. One of the constant problems about copyright law is that you can rarely be certain about things: you can only really know as a result of a law suit — and law suits cost more than the bone of contention is usually worth.
Publishers contracting out jacket design to freelance designers should no-doubt note somewhere in their communications with the designer that the result will be considered work made for hire. No way you want to be delaying a reprint gettting permission for a copyright holder.