Open English Wordnet
Adjectives
(s)elusive, subtledifficult 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” (s)subtleable to make fine distinctions“a subtle mind” (s)insidious, pernicious, subtleworking or spreading in a hidden and usually injurious way“glaucoma is an insidious disease”,
“a subtle poison”