tertium quid

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Something that cannot be classified into either of two groups considered exhaustive; an intermediate thing or factor.
2. n. A third person or thing of indeterminate character.

Century Dictionary and Cyclopedia

1. Something neither mind nor matter; especially, an idea regarded as not a mere modification of the mind nor a purely external thing in itself.
2. Hence Something mediating between essentially opposite things.

Wiktionary

1. n. something intermediate between two classifications

GNU Webster's 1913

1. A third somewhat; something mediating, or regarded as being, between two diverse or incompatible substances, natures, or positions.

WordNet 3.0

1. n. some third thing similar to two opposites but distinct from both