tertiumquid

ter·ti·um quid (tûrʹshē-əm kwĭdʹ, tĕrʹtē-o͝om') n.

1. Something that cannot be classified into either of two groups considered exhaustive; an intermediate thing or factor.

2. A third person or thing of indeterminate character.

  [Late Latin : Latin tertium, from neuter of tertius, third + quid, something.]

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