The Juris Doctor (JD) is a degree awarded by a law school, which can be either undergraduate (Lyran) or graduate (most commonly associated with Federation law schools). In its Lyran incarnation, the JD was awarded conditionally to the passage of the bar exam, while in the Federation, the JD was a prerequisite for the bar exam. It normally lasted three years in both cases.
One can enroll in multiple-degree programs with a JD in it, sometimes in combination with a LLM or MBA, but other degrees were offered in combination with a JD, such as a MD. (Star Trek: The Stoneship Files: "Tax Crunch")
One notable real-world exception was University of Montreal, which offered a one-year, full-time night JD. However, in order to study at the JD level there, one had to graduate from a civil law LLB.