imperceptible
ɪmpəˈsɛptɪb(ə)l
adjective
so slight, gradual, or subtle as not to be perceived.
his head moved in an almost imperceptible nod
unnoticeable,undetectable,indistinguishable,indiscernible,unapparent,inappreciable,invisible,inaudible,impalpable,unobtrusive,impossible to detect,slight,small,subtle,faint,fine,inconsequential,negligible,tiny,minute,minuscule,microscopic,nanoscopic,infinitesimal,indistinct,unclear,obscure,vague,indefinite,shadowy,hard to make out