In psychophysics, the just noticeable difference (usually abbreviated as jnd, using lowercase letters) is the smallest difference in a sensory input that is perceivable by a human being or other animal. It is also known as the differential threshold.

For many sensory dimensions, the jnd is an increasing function of the base level of input, and the ratio of the two is roughly constant. Measured in physical units, we have

,

where I is the original intensity of stimulation, ΔI is the addition to it required for the difference to be perceived (the jnd), and k is a constant. This rule was first discovered by E.H. Weber, in experiments on the thresholds of perception of lifted weights, and it is therefore known as Weber's law ; the constant k is called the Weber constant. It is true, at least to a good approximation, of many but not all sensory dimensions, for example the brightness of lights, and the intensity and the pitch of sounds. It is not true, however, of the wavelength of light. S. S. Stevens argued that it would hold only for what he called prothetic sensory continua, where change of input takes the form an increase in intensity or something obviously analogous; it would not hold for metathetic continua, where change of input produces a qualitative rather than a quantitative change of the percept.

The jnd is a statistical, rather than an exact quantity: from trial to trial, the difference that a given person notices will vary somewhat, and it is therefore necessary to conduct many trials in order to determine the threshold. The jnd usually reported is the difference that a person notices on 50% of trials. If a different proportion is used, this should be included in the description—for example one might report the value of the "75% jnd".

Modern approaches to psychophysics, for example signal detection theory, imply that the observed jnd, even in this statistical sense, is not an absolute quantity, but will depend on situational and motivational as well as perceptual factors.