1) Barely audible 2) Barely visible 3) French word used in English 4) Immaterial 5) Impalpable 6) Inaudible 7) Incognizable 8) Incognoscible 9) Indiscernible 10) Indistinguishable 11) Insensible 12) Insignificant 13) Invisible 14) Microscopic 15) Minute 16) Negligible 17) Not perceptible 18) Subliminal 1) Inappreciable 2) Inaudible 3) Incognizable 4) Incognoscible 5) Indescribable 6) Indiscernible 7) Indistinguishable 8) Invisible 9) Subliminal 10) Unhearable 11) Unobservable 12) Unperceivable 13) Unseeable 
• (a.) Not perceptible; not to be apprehended or cognized by the souses; not discernible by the mind; not easily apprehended.
Im`per·cep'ti·ble adjective [ Prefix im- not + perceptible : confer French imperceptible .] Not perceptible; not to be apprehended or cognized by the senses; not discernible by the mind; not easily apprehended. « Almost imperceptible to the touch.» Dryden...
[adj] - impossible or difficult to perceive by the mind or senses
adjective impossible or difficult to perceive by the mind or senses; `an imperceptible drop in temperature`; `an imperceptible nod`; `color is unperceivable to the touch`
very slight, gradual, or subtle: the imperceptible slope of the road. · not perceptible; not perceived by or affecting the senses.
[SAT terms] impossible or difficult to sense