Origin: Latin prefix sub-
Subtle has 4 different meanings across 1 category:
working or spreading in a hidden and usually injurious way
"glaucoma is an insidious disease"
"a subtle poison"
Hard to grasp; not obvious or easily understood; barely noticeable.
"The subtle difference between the two shades of blue was nearly impossible to spot in natural light."
In plain English: Subtle means something is very small, quiet, or not obvious at first glance so you have to look closely to notice it.
"The subtle difference between their two voices was hard to notice at first."
Usage: Use subtle to describe changes, differences, or qualities that are delicate and difficult to notice immediately. Avoid confusing it with words like slight when you specifically mean something is nuanced rather than just small in degree.
The word "subtle" comes from Middle English sotil and originally described a person's mind as being clever or ingenious rather than just meaning delicately made today. It traveled into modern usage while retaining its core sense of sharpness but shifting to emphasize fine distinctions over cunning behavior.