The absolute threshold is the weakest level of a stimulus that a person can detect half the time. Below that level the stimulus is usually missed; above it detection becomes reliable. The concept comes from nineteenth-century psychophysics and remains a basic measure of sensory sensitivity.

Thresholds differ across the senses and across individuals. A candle flame on a clear night can be seen from thirty miles away under ideal conditions; a watch ticking can be heard from twenty feet in a quiet room. Taste and smell thresholds vary with the chemical and with the state of the observer. Age, fatigue, attention, and expectation all shift the measured value. Modern testing often uses signal-detection methods that separate true sensitivity from the observer’s willingness to say “yes.”

The 50-percent criterion is conventional. It acknowledges that detection is probabilistic rather than all-or-none. A stimulus at threshold is not invisible or inaudible half the time in any simple sense; rather, the internal response it produces is sometimes strong enough to cross the decision criterion and sometimes not.

Knowing the absolute threshold helps engineers design displays and alarms, and it gives psychologists a quantitative starting point for studying how sensation becomes perception.