just noticeable difference

just no·tice·able dif·fer·ence

noun
\ˌjəst-ˌnōt-ə-sə-bəl-ˈdif-(ə-)rən(t)s\

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