The difference threshold is the smallest difference between two stimuli that a person can detect reliably. It is also called the just noticeable difference, or jnd.

Ernst Weber observed that the difference threshold is not a fixed amount but a constant proportion of the original stimulus. To notice an increase in the weight of a backpack, the added load must be a certain percentage of the existing weight; the same absolute addition that is obvious for a light pack is undetectable for a heavy one. This relationship is known as Weber’s law. The exact proportion varies across sensory modalities and intensity ranges.

Modern psychophysics measures the difference threshold with careful methods that account for response bias. Signal-detection approaches separate true sensitivity from the observer’s willingness to report a difference. Practical applications appear in product design, quality control, and the calibration of displays and audio equipment.

The difference threshold marks the boundary between change that is noticed and change that passes without awareness.