subtle

Adjective

• working or spreading in a hidden and usually injurious way
  • glaucoma is an insidious disease
    
  • a subtle poison
    

• able to make fine distinctions
  • a subtle mind
    

• 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
    

Related words

• Synonyms: 
  insidious , pernicious , elusive