Imperceptible /(?)/

Im·per·cep·ti·ble

Imperceptible

a.
  1. Not perceptible; not to be apprehended or cognized by the senses; not discernible by the mind; not easily apprehended.
    Almost imperceptible to the touch.
    Its operation is slow, and in some cases almost imperceptible.
    Their . . . subtilty and imperceptibleness.