Final answer:
The smallest difference between two stimuli that is detectable 50 percent of the time is known as the just noticeable difference (jnd) or difference threshold.
Step-by-step explanation:
The smallest difference between two stimuli that is detectable 50 percent of the time is called the just noticeable difference (jnd) or difference threshold. This term, rooted in psychophysics, relates to the ability of a sensory system to detect changes or differences between two stimuli. The concept of the just noticeable difference is based on Ernst Weber's work from the 1830s and further developed into Weber's law, which posits that the difference threshold is a constant fraction of the original stimulus. This fraction varies depending on the sensory modality involved.
An example illustrating the just noticeable difference could be in the context of sound: if you were to increase the volume of music that's already playing softly in a room, there would be a certain increment at which you would first notice the volume change. This increment at which the change is detected is the just noticeable difference for the volume of the music. According to Weber's law, if the original volume were louder, you'd need a larger increase in volume to reach the difference threshold for noticing a change.