that the angular separation of the two objects, as measured (in radians) at the aperture, should be at least 
where 
is the wavelength of the radiation used. [P6.4]
is: an arbitrary criterion establishing the condition in which the optical images of two point-like objects can be said to be resolved by the optical system that created them [P6.4]
requires: that the central diffraction maximum of one image should fall on or beyond the first diffraction minimum of the other. [P6.4]
implies: for a circular aperture of diameter that the angular separation of the two objects, as measured (in radians) at the aperture, should be at least where is the wavelength of the radiation used. [P6.4]
Copyright 1997, The Open University