Englishimperceptible
adjective
the change was slow and imperceptible:
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.
▷antonyms obvious, noticeable.
imperceptible Oxford Dictionary of English