A doctor was a title that denoted that someone was either a medical physician or a highly educated individual.
When John Crichton was experiencing visions of being on Earth forced on him by the Scarrans, Aeryn Sun was in the role of a medical doctor named Bettina Fairchild, while Zotoh Zhaan played the role of a doctor of Psychiatry named Jean Kaminski. ("Won't Get Fooled Again")
The "Alien Visitation" show viewed by John Crichton featured a number of interviews with educated individuals including doctors of Psychology (Edith Anderson), Sociology (Jason Fletcher), Anthropology (Garrett Hamilton), Cognitive Behaviorism (Edmund Johnston), Criminal Psychology (Jayne O'Connor), and Xenobiology (Adrian Walker). ("A Constellation of Doubt")