Professors in the United States commonly occupy any of several positions of teaching and research within a college or university. In the U.S., the word "professor" informally refers collectively to the academic ranks of assistant professor, associate professor, or professor. This usage differs from the predominant usage of the word professor internationally, where the unqualified word professor only refers to "full professors." The majority of university lecturers and instructors in the United States, as of 2015, do not occupy these tenure-track ranks, but are part-time adjuncts, or more commonly referred as college teachers.