A professor, informally often known as full professor, is the highest academic rank at universities and other post-secondary education and research institutions in most countries. Literally, professor derives from Latin as a "person who professes" being usually an expert in arts or sciences, a teacher of the highest rank. In some countries, the word professor is also used in titles of lower ranks such as associate professor and assistant professor.Professors conduct original research and commonly teach undergraduate, graduate, and/or professional courses in their field of expertise. In universities with graduate schools, professors may mentor and supervise graduate students who are conducting research for a thesis or dissertation. Professors typically hold a Ph.D., another doctorate or a different terminal degree. Some professors hold a master's degree or a professional degree such as an MD as their highest degree.