Imperceptible (adjective)

1. Not able to be perceived by the senses; so slight as to be unnoticeable.
2. So gradual as to be almost unnoticeable; almost imperceptible.
3. Not easily perceived or noticed; subtle.

Origin:

From the latin word "imperceptibilis" meaning "not able to be perceived".

Examples:

1. The change in temperature was imperceptible to the human eye.
2. The sound of the clock ticking was imperceptible in the quiet room.
3. The fading of colors on the painting was imperceptible to most observers.
4. The shift in his mood was imperceptible, but she could tell something was wrong.
5. The imperceptible increase in weight was almost impossible to notice.