Which term describes something that is impossible to perceive?

The term that describes something that is impossible to perceive is "imperceptible." This word specifically refers to things that are so small, subtle, or indistinguishable that they cannot be detected or noticed by the senses. It encompasses a broader range of perceptions, including sight, sound, taste, touch, or smell, indicating that something may exist but cannot be registered by any of these senses.

In contrast, "invisible" strictly refers to something that cannot be seen. While it may imply a lack of visibility, it doesn't necessarily address other forms of perception, such as sounds or smells.

"Inaudible" specifically pertains to sounds that cannot be heard, thus limiting its application to auditory perception alone. "Unnoticed," while it can imply that something has been overlooked or ignored, does not inherently convey the idea of being imperceptible; something may be noticed but not given attention, or it may have been entirely perceivable but disregarded by the observer.

Therefore, "imperceptible" is the most accurate term for describing something that cannot be perceived by any means.