Originally: an open question, something which has not yet been determined or resolved. Later: (Law) a case or question not covered by existing law or precedent.
res integra/reɪz ˈɪntəɡrə/ /rɛz ˈɪntəɡrə/
Early 17th century; earliest use found in Journal of the House of Commons. From classical Latin rēs integra question or issue (also specifically a lawsuit) which has not yet been decided from rēs thing + integra, feminine of integer.