tertium quid

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun something intermediate between two classifications

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun some third thing similar to two opposites but distinct from both