Diffraction-limited
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The ability to produce optical images with angular separations as small as the instrument's theoretical limit. A telescope with this capability is said to be diffraction limited.
In astronomy, a diffraction-limited observation is one that is only limited by the optical power of the instrument used, as opposed to seeing-limited, where the main limit is given by atmospheric effects.
The resolution of a given instrument is proportional to the diameter of its objective, and inversely proportional to the wavelength of the light being observed. Diffraction effects are the main contributors to this limit.
Most telescopes on the Earth at ground level work at a much lower resolution than diffraction-limited, because of the distortion introduced by the passage of light through several kilometres of atmosphere. Some advanced observatories have recently started using adaptive optics technology, resulting in ever greater resolution as the technology improves, but it's still difficult to reach the diffraction limit. Radiotelescopes are better in this regard, because the wavelengths they use (from millimeters to meters) are so long that the atmospheric distortion is negligible.
Space-based telescopes (such as the HST, or a number of non-optical telescopes) always work at their diffraction limit.