The Collaborative International Dictionary of English v.0.48:

Altern \Al"tern\, a. [L. alternus, fr. alter another: cf. F.
   alterne.]
   Acting by turns; alternate. --Milton.
   [1913 Webster]

   Altern base (Trig.), a second side made base, in
      distinction from a side previously regarded as base.
      [1913 Webster]