IMPERCEPTIBLE

adj. 1 invisible, indiscernible, indistinguishable, undetectable, obscure, vague, ill-defined; inaudible The differences between the twins are imperceptible 2 indistinct, unclear, unnoticeable, slight, subtle, inconsiderable, inappreciable, minute, tiny, slight, minuscule, infinitesimal, microscopic Her only response was an imperceptible flicker of her eyelid