<p><strong>Adjective</strong></p><ul><li>Very slight, gradual, or subtle: the imperceptible slope of the road.</li><li>Not perceptible; not perceived by or affecting the senses.</li></ul><p><strong>Synonyms</strong></p><ul><li>Undetectable</li><li>Unperceivable</li><li>Hidden</li></ul><p><strong>Noun</strong></p><ul><li>Something not capable of being perceived by the senses: metaphysical speculation about imperceptibles.</li></ul>