From AMS Glossary
The minimum contrast Cthresh at which an observer can just distinguish a target object from its surroundings.
Threshold contrast depends on target angular size θ, the surrounding luminance Ls, and the desired probability of detection (usually 50%–99%). For a given Ls, Cthresh has a minimum value at some optimal θ (i.e., the target size that is most easily detected). Threshold contrast also varies among observers and across time for a given observer. Because detection outdoors primarily depends on luminance rather than chromaticity differences, Cthresh is usually calculated from spectrally integrated luminances. For alerted observers (i.e., those expecting a target to appear) and daytime luminance levels, Cthresh ranges from ∼0.005 to 5.0 or more. Unalerted observers may require Cthresh values that are five or more times greater than those for alerted observers.