A Subtlety

(uncountable) The quality of being subtle.
1. (of things) The quality of being scarcely noticeable or difficult to discern.
2. (of things) The quality of being done in a clever way that is not obvious or not direct; the quality of being carefully thought out.

1. (of people) The quality of being able to achieve one's aims through clever, delicate or indirect methods. Synonyms: discretion, finesse, savoir-faire
2. (of people) The quality of being able to notice or understand things that are not obvious. Synonyms: acumen, perceptiveness, perspicacity

2 . (countable) An instance of being subtle, a subtle thing, especially a subtle argument or distinction.

(countable, historical) An ornate medieval illusion dish or table decoration, especially when made from one thing but crafted to look like another.
(uncountable, countable, archaic) The quality of being clever in surreptitious or deceitful behaviour; an act or argument that shows this quality. Synonyms: artifice, craftiness, cunning, deceitfulness, slyness, trickery
(countable, obsolete) A trick that creates a false appearance. Synonyms: deception, illusion
(uncountable, obsolete) The property of having a low density or thin consistency.
(uncountable, obsolete) The property of being able to penetrate materials easily. Synonyms: penetrancy, piercingness