* The institution: Prestigious universities and research-intensive institutions almost always require a PhD. Smaller colleges or community colleges may sometimes hire instructors with only a Master's degree, especially for certain subjects or if they have significant professional experience.
* The position: A tenure-track position almost always requires a PhD. However, adjunct professors, lecturers, or instructors might only need a Master's degree, particularly for teaching introductory courses.
* The subject: Some fields might be more flexible than others.
* The country: Requirements vary internationally.
In short, while a Master's degree can be sufficient for *some* university teaching positions, a PhD is far more common and generally necessary for career advancement and long-term employment in academia.