Answer: invisible

AnswerCrossword Clue
INVISIBLEImperceptible.
INVISIBLEUnseen
INVISIBLELike a spy's ink?
invisibleUnable to be seen; not visible to the eye
invisible(of a person) Treated as if unable to be seen; ignored or not taken into consideration
invisibleConcealed from sight; hidden
invisiblethat cannot be seen / an invisible export
invisibleRelating to or denoting earnings that a country makes from the sale of services or other items not constituting tangible commodities