In a previous book, Dr. Berry said, he has a page listing all the terms that have been used for contingent faculty: One of them is “uncompensated.”

The union representing contingent faculty at the University of California has been fighting the uncompensated positions for years, said Mia McIver, the president of the union, which represents about 7,000 members. “The fact that it is common does not excuse it,” she said.

The union suspects that the number of uncompensated teachers at the university is increasing, said Dr. McIver, who is also a lecturer in the U.C.L.A writing program. “As of March 2019, we had identified 26 faculty members at U.C.L.A. alone,” she said.

In the California system, the trend seemed to have begun with the financial crisis of 2008, Dr. McIver said. By 2010, she said, “We became aware of people who had been laid off and who were teaching for free in the hopes, without any commitment from the university, that if the work came back they would be hired back to teach for pay.”

The union won a settlement with the administration in 2016 requiring compensation for lecturers, who are mostly part-time and make up a majority of contingent faculty, Dr. McIver said. But while lecturers are now unionized, adjuncts are not, allowing the university to have adjunct positions known as “zero percent appointments,” meaning that they are unpaid.

A spokesman for U.C.L.A., Steve Ritea, said that before the settlement, the people who taught for free were often full-time professionals with other income. He said he could not comment on the number of zero percent appointments without seeing the documents the union was relying on. But he said that a typical example of a zero percent adjunct is a tenured professor at another institution who has a formal affiliation with U.C.L.A. that might include mentoring students or serving on committees. Or someone who has moved to another university but wants to finish out a grant or a project.