The "diffraction limit" refers to the fundamental limit on the resolution of optical systems, such as microscopes and telescopes. It occurs because light behaves as a wave, and when it passes through a small aperture or around an object, it spreads out, creating a pattern of interference. This spreading prevents us from distinguishing between two closely spaced points. In practical terms, the diffraction limit means that there is a minimum distance between two points that can be resolved clearly. For example, in a telescope, this limit affects how well we can see distant stars or planets. Similarly, in a microscope, it determines how small the details we can observe in a cell or tissue sample can be.