Tenure is the right of a professor not to be subjected to summary dismissal without just cause; this right is typically awarded contractually after a probationary period. Some positions in academic institutions are considered "tenure-track," which means that the holder of the position is guaranteed eventual consideration for tenure. Non-tenure-track positions will generally not be considered for tenure.
The typical entry-level rank for faculty on the tenure-track is Assistant Professor, although this depends on the field of study and the institution. After spending 6-8 years at the rank of Assistant Professor, individuals will either earn the rank of Associate Professor, which usually indicates that the individual has been granted tenure at the university, or their employment will be terminated at most institutions. This is referred to "up or out," meaning that after 6-8 years, a tenure-track Assistant Professor must either be awarded tenure or he or she will be dismissed. After receiving tenure, typically, promotion to "full" Professor takes another six to ten years for an Associate Professor.
Some universities or institutions may provide unique titles to senior faculty members who have achieved extraordinary accomplishments in their field. These may be a "named chair" or "named professorship," which may include funds from the endowment for the professor's usage. Some other research institutions may provide the title of "Distinguished Professor" to indicate that the recipient has made large contributions to the university or to his or her academic field. However, many institutions have a highest rank of Professor.
On the other hand, instructors not on the tenure-track may be referred to by a variety of names, including Clinical Professor, Professor of Practice, Research Professor, Research Associate, Lecturer, and Instructor. These individuals may teach full-time and have administrative duties within the institution, but they generally do not have research obligations. Contrary to popular belief, Lecturers and Instructors typically hold advanced graduate or professional degrees, just like full Professors.
Some individuals who are not tenure-track may have titles preceded by the word "Adjunct," which generally means part-time status. However, currently, many doctorate-holders seek work through holding multiple adjunct jobs, with the end goal of entering the tenure-track.