Adjective
1. be difficult to detect or grasp by the mind;
"his whole attitude had undergone a subtle change""a subtle difference""that elusive thing the soul"2. faint and difficult to analyze;
"subtle aromas"3. able to make fine distinctions;
"a subtle mind"4. working or spreading in a hidden and usually injurious way;
"glaucoma is an insidious disease""a subtle poison"