This is usually referred to as the Doctor of Philosophy, or Ph.D. In most academic fields, it is the highest degree possible. Most university professors and scientific researchers hold a Ph.D.
The Doctor of Philosophy is so named out of tradition. The degree applies to most academic disciplines, regardless of whether they are related to philosophy.
The best known professional doctoral degrees are the Doctor of Medicine (M.D.) and the Juris Doctorate (J.D.) The M.D. is the degree earned by physicians, while the J.D. is the basic law degree.
Many universities award honorary doctoral degrees, usually to formally recognize a noted individual for exceptional achievement in or contribution to his or her field.
The time required to earn a doctorate varies by degree and field. Law students generally complete a J.D. in three years. The M.D. generally requires four years of study. A Ph.D. can require four to 10 years to complete, depending on the school and field of study.