elusive

Adjective

• making great mental demands; hard to comprehend or solve or believe
  • a baffling problem
    
  • I faced the knotty problem of what to have for breakfast
    
  • a problematic situation at home
    

• difficult to detect or grasp by the mind or analyze
  • his whole attitude had undergone a subtle change
    
  • a subtle difference
    
  • that elusive thing the soul
    

• skillful at eluding capture
  • a cabal of conspirators, each more elusive than the archterrorist- David Kline
    

• difficult to describe
  • a haunting elusive odor
    

Related words

• Synonyms: 
  baffling , knotty , problematic , problematical , tough , subtle