just noticeable difference
noun
just no·tice·able dif·fer·ence
| \ ˌjəst-ˌnōt-ə-sə-bəl-ˈdif-(ə-)rən(t)s
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Medical Definition of just noticeable difference
: the minimum amount of change in a physical stimulus required for a subject to detect reliably a difference in the level of stimulation