tertium quid

from WordNet (r) 3.0 (2006)

tertium quid
    n 1: some third thing similar to two opposites but distinct from
         both

from The Collaborative International Dictionary of English v.0.48

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.
   [Webster 1913 Suppl.]