It is closest distance between two points at which the point can be seen clearly & separate as entities. It is ability to distinguish two objects as a separate entities.
R.P = _lamba___
The smaller value for resolving power, the smaller object can be seen distinctly. The highest resolution possible in a compound microscope is about 0.22 micrometer. This means that two object closer than 0.22 micrometer aren't resolvable as distinct & separate. Shorter the wavelength of illuminating the specimen, greater is the resolving power, better the resolving power so that smaller objects can be viewed.