Imperceptible

So slight, gradual, or subtle as to be difficult or impossible to perceive or detect. It describes changes or qualities that escape notice.

From Latin 'imperceptibilis,' composed of 'in-' (not) and 'perceptibilis' (able to be perceived), from 'percipere' (to perceive). The word entered English in the 17th century, emphasizing the limits of human perception.