The distance between two objects at which it first becomes impossible to resolve them as separate.
The limit of resolution is generally represented with the symbol "D". In the context of microscopy and vision, the limit of resolution inherent to a particular device is defined as follows:
D = (.61 (lambda)) / (N sin a)

Where lambda is the wavelength of light used (smaller is better; limit of D for visible light is about 200 nanometers),
N is the refractive index of the material through which the light is passing (refractive index of air is one, more dense media have higher N and therefore better resolution (which is why many microscopes use between the lense and specimen)),
and a is the angle or aperture, or 1/2 the angle of the cone of light produced.