Unable to be noticed or felt because of being very slight. (Cambridge Dictionaries)
Not perceptible by a sense or by the mind : extremely slight, gradual, or subtle. (Merriam – Webster)
Very slight, gradual, or subtle: (Dictionary.com)
Not perceptible; not perceived by or affecting the senses. (Dictionary.com)
So slight, gradual, or subtle as not to be perceived. (Oxford Dictionaries)
Something that is imperceptible is so small that it is not noticed or cannot be seen. (Collins Dictionary)
NASA Pursues Atom Optics to Detect the Imperceptible.
Molar and molecular mobilities: The politics of perceptible and imperceptible movements.
How imperceptible can orthodontics be really?
An imperceptible and robust audio watermarking algorithm.