| adjective | difficult to detect or grasp by the mind or analyze “his whole attitude had undergone a subtle change” |
| adjective | able to make fine distinctions “a subtle mind” |
| adjective | working or spreading in a hidden and usually injurious way “glaucoma is an insidious disease” |
| elude | verb | be incomprehensible to; escape understanding by |
| elusiveness | noun | the quality of being difficult to grasp or pin down |
| insidiousness | noun | subtle and cumulative harmfulness (especially of a disease) |
| perniciousness | noun | grave harmfulness or deadliness |