资料来源 : Webster's Revised Unabridged Dictionary (1913)

Tertium quid \Ter"ti*um quid\ [L.]
   A third somewhat; something mediating, or regarded as being,
   between two diverse or incompatible substances, natures, or
   positions.

资料来源 : WordNet®

tertium quid
     n : some third thing similar to two opposites but distinct from
         both