[adj] faint and difficult to analyze; "subtle aromas"
[adj] working or spreading in a hidden and usually injurious way; "glaucoma is an insidious disease"; "a subtle poison"
[adj] 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"
[adj] able to make fine distinctions; "a subtle mind"