The resolving power of the eye.
When the separation between the incoherent point sources is decreased, in the limit a decision whether the image distribution arises from one or two points or a short bar rests, in the absence of prior information, on the detection of an indentation. That, in essence, is the concept of resolution.
Westheimer
Gerald
G
D004192:Discrimination, Psychological; D006801:Humans; D012160:Retina / Q000502:physiology; D009626:Terminology as Topic; D014792:Visual Acuity / Q000502:physiology*