Something that is imperceptible is so small that it is not noticed or cannot be seen. Brian's hesitation was almost imperceptible. + imperceptibly im·per·cep·tibly The disease develops gradually and imperceptibly. almost impossible to see or notice
impossible or difficult to perceive by the mind or senses; "an imperceptible drop in temperature"; "an imperceptible nod"; "color is unperceivable to the touch"
{s} not perceptible, undetectable, not discerned by the senses; subtle, slight, unnoticeable
Not perceptible; not to be apprehended or cognized by the senses; not discernible by the mind; not easily apprehended