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.]

The Collaborative International Dictionary of English v.0.48:

Extern \Ex*tern"\, a. [Cf. F. externe. See External.]
   External; outward; not inherent. [Obs.] --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Extern \Ex*tern"\, n. [Cf. F. externe.]
   1. A pupil in a seminary who lives without its walls; a day
      scholar.
      [1913 Webster]

   2. Outward form or part; exterior. [R.]
      [1913 Webster]

   3. same as externe.
      [PJC]

WordNet (r) 3.0 (2006):

extern
    n 1: a nonresident doctor or medical student; connected with a
         hospital but not living there [syn: extern, medical
         extern]