In the field of psychophysics, that branch of experimental psychology that studies sensation and perception, a jnd is the amount that something must be changed for the difference to be noticeable, defined to mean that the change is detectable half the time. My goal is to make a noticeable difference -- many jnds worth -- in human-centered technology.
I started my career as an experimental/mathematical psychologist in psychophysics, and my love of the exquisite sensitivity and dynamic range of hearing and seeing, as well as the power of human perception has stayed with me.
For the definition of jnd as used in psychophysics, see the Wikipedia definition.