The Collaborative International Dictionary of English v.0.48:

Externe \Ex`terne"\, Extern \Ex*tern"\([e^]ks`t[e^]rn"), n. [F.
   Cf. Extern.]
   a person affiliated with an institution who does not reside
   there; especially, (Med.) a doctor or medical student who is
   in attendance upon, or is assisting at, a hospital, but who
   does not reside in it.
   [Webster 1913 Suppl.]