The Collaborative International Dictionary of English v.0.48:

Obversion \Ob*ver"sion\ ([o^]b*v[~e]r"sh[u^]n), n. [L. obversio
   a turning towards.]
   1. The act of turning toward or downward.
      [1913 Webster]

   2. (Logic) The act of immediate inference, by which we deny
      the opposite of anything which has been affirmed; as, all
      men are mortal; then, by obversion, no men are immortal.
      This is also described as "immediate inference by
      privative conception." --Bain.
      [1913 Webster]