Learner's definition of SUBTLE
[or more subtle; most subtle]
1 : hard to notice or see : not obvious
a subtle difference in meaning between the words
the subtlest details
Racial discrimination still exists, only now it's subtler than it once was.
2 : clever and indirect : not showing your real purpose
When it comes to giving criticism, sometimes it's best to take a subtle approach.
He didn't seem to understand my subtle hints.
It was her subtle way of telling me to mind my own business.
3 : having or showing skill at recognizing and understanding things that are not obvious : perceptive
She has a subtle mind.
a subtle analysis