* Not a first degree: Many people pursue a JD *after* earning a bachelor's degree (a first degree). It's a postgraduate professional degree.
* Not strictly a terminal degree: While many JD holders don't pursue further academic degrees, it's possible to continue on to an LLM (Master of Laws) or SJD (Doctor of Juridical Science), which are considered terminal degrees in the field of law. Therefore, a JD can be a stepping stone to further legal scholarship.
So the best answer is that it's a professional degree that typically follows an undergraduate degree and can be followed by further postgraduate legal studies.