elusive

adjective

1. 1

   skillful at eluding capture

   a cabal of conspirators, each more elusive than the archterrorist"- David Kline

2. 2

   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

3. 3

   difficult to describe

   a haunting elusive odor

4. 4

   difficult to detect or grasp by the mind or analyze

   his whole attitude had undergone a subtle change

   a subtle difference