The fundamental physical limit to how sharply a telescope can focus light, determined by the wave nature of light and the telescope's aperture diameter. No telescope can achieve resolution better than this theoretical limit, which equals approximately 1.22 times the wavelength divided by the aperture diameter.
From Latin 'diffractus' (broken apart) describing how waves bend around obstacles, combined with 'limit' from Latin 'limes' (boundary). The concept emerged from 19th-century wave optics theory, particularly the work of Lord Rayleigh who established the mathematical relationship between aperture size and ultimate resolution.
The diffraction limit is nature's ultimate speed limit for telescope resolution - even a perfect telescope in space can't break this barrier because light itself has wave properties! This is why radio telescopes need to be enormous compared to optical telescopes: radio waves are thousands of times longer than visible light, so you need a proportionally larger dish to achieve the same sharpness.
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