The Collaborative International Dictionary of English v.0.48:

Metabasis \Me*tab"a*sis\, n.; pl. Metabases. [NL., fr. Gr. ?,
   fr. ? to pass over; ? beyond, over + ? to go.]
   1. (Rhet.) A transition from one subject to another.
      [1913 Webster]

   2. (Med.) Same as Metabola.
      [1913 Webster] Metabola