Just Noticeable Difference is a concept that describes the smallest amount of change that can be perceived by the human senses. It is used in many fields, such as psychology, engineering, and marketing, to measure the effectiveness of a product or service. It is based on the idea that people can
From Wikipedia
In psychophysics, a just-noticeable difference (JND) is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time. This limen is also known as the difference limen, difference threshold, or least perceptible difference.