Final answer:
The smallest stimulus level that can just be detected is known as the absolute threshold, which is the minimum stimulus energy required to be detected 50% of the time.
Step-by-step explanation:
The smallest stimulus level that can just be detected is referred to as the absolute threshold. This is the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time. It is different from the difference threshold or just noticeable difference (jnd), which is the amount of change in a stimulus required to detect a difference. The concept of absolute threshold is a fundamental part of understanding how the human sensory system operates, and it is crucial in fields such as psychology, sensory science, and neuroscience.