Subtle

1. adjective

   working or spreading in a hidden and usually injurious way

   “glaucoma is an insidious disease”

   Similar: insidious, pernicious

2. adjective

   difficult to detect or grasp by the mind or analyze

   “his whole attitude had undergone a subtle change”

   Similar: elusive

3. adjective

   able to make fine distinctions

   “a subtle mind”