1. in psychophysics, the magnitude of a stimulus that will lead to its detection 50% of the time.

2. the minimum intensity of a stimulus that is necessary to evoke a response. For example, an auditory threshold is the slightest perceptible sound; an excitatory threshold is the minimum stimulus intensity that triggers an action potential in a neuron; and a renal threshold is the concentration of a substance in the blood required before the excess is excreted. Also called limen; response threshold. See also absolute threshold; difference threshold.