Elusive

1. adjective

   making great mental demands; hard to comprehend or solve or believe

   “a baffling problem”

   Similar: baffling, knotty, problematic, problematical, tough

2. adjective

   difficult to detect or grasp by the mind or analyze

   “his whole attitude had undergone a subtle change”

   Similar: subtle

3. adjective

   skillful at eluding capture; - David Kline

   “a cabal of conspirators, each more elusive than the archterrorist”

4. adjective

   difficult to describe

   “a haunting elusive odor”