Angular Resolution is a measure of the ability of a telescope (optical, Radio, etc.) to distinguish spatial detail. "Angular" refers to the measure describing an angle between two distinguishable features of an image with the observer as the vertex of the angle.
The physics of Diffraction determines a minimum angular resolution of a telescope based upon the size of its Aperture. A point of light becomes a disk surrounded by rings if light (known as an Airy Disk), and if the angle between two such points is sufficiently small, the disks are not distinct.
Lord Rayleigh's Rayleigh criterion describes a telescope's resolution by the distance between two points such that one's disk maximum coincides with the adjacent disk's minimum, which the following formula calculates:
λ θ = 1.220 ——— D
Where θ is the angular resolution (radians), λ is the Wavelength, and D is the aperture diameter (both in same unit, e.g., meters).
With multiple telescopes arranged as an Interferometer, smaller angular resolution can be obtained. Also, computer-based analysis on the overlapping Airy disks can sometimes identify and locate individual sources, achieving higher angular resolution than the Rayleigh criterion would imply. The term Speckle Suppression includes this and other methods of surpassing the Rayleigh criterion.
Example angular resolutions:
(Spektr-R's 10m dish limits it to bright Radio Sources.)
Atacama Cosmology Telescope (ACT)
Galactic All-Sky Survey (GASS)
Stellar Distance Determination
Very-long-baseline Interferometry (VLBI)