(psychophysics) the difference between two stimuli that (under properly controlled experimental conditions) is detected as often as it is undetected WordNet 3.0 & Open English WordNet

In the branch of experimental psychology focused on sense, sensation, and perception, which is called psychophysics, a just-noticeable difference or JND is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time. Wikipedia

The amount something must be changed in order for a difference to be noticeable Wikipedia Disambiguation

Amount a stimulus must be changed to be detected Wikidata