This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Impossible or difficult to perceive by the mind or senses: an imperceptible drop in temperature.

adj. So subtle, slight, or gradual as to be barely perceptible: an imperceptible nod.

adj. Not perceptible, not detectable, too small in magnitude to be observed.

adj. Not perceptible; not to be apprehended or cognized by the senses; not discernible by the mind; not easily apprehended.

Not perceptible; that cannot be perceived.

Incapable by nature of affecting the senses.

Too minute, fine, gradual, subtle, or evanescent to be discerned by the senses; producing an excitation of the nerves less than the threshold of sensation. See threshold.

n. That which cannot be perceived with the naked eye, or realized by sensation.

adj. impossible or difficult to perceive by the mind or senses

"My brother rose to do so but he saw neither ewer nor basin; yet his host kept washing his hands with invisible soap in imperceptible water and cried,"

indiscernible    undetectable    incognizable    impalpable    subliminal    unobservable    incognoscible    insensible   

perceptible   

imperceptible increase   

unsensible    indistinguishable    insensible    imperceivable    inappreciable    unperceivable    inapprehensible    indiscernible    undiscoverable    inaudible   

perceptible    infinitesimal    upward    gradual    slight    involuntary    convulsive    inaudible    telltale    abrupt