The Oxford Thesaurus An A-Z Dictionary of Synonyms - imperceptibleadj.

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.