WordWeb Online

1. Difficult to describe
   "a haunting elusive odour"
    
2. Skilful at eluding capture
   "a cabal of conspirators, each more elusive than the arch terrorist"
    
3. Difficult to detect or grasp by the mind or analyse
   "that elusive thing the soul"
   - subtle
    
4. Making great mental demands; hard to comprehend or solve or believe
   - baffling, knotty, problematic, problematical, tough

Sounds like: illusive, allusive

See also: artful, difficult, elude, elusiveness, hard, impalpable, unidentifiable