The term "ultraviolet catastrophe" refers to a paradox that emerged in scientists' understanding of the way light is emitted by black bodies that exist in thermal equilibrium. According to the classical understanding of physics, a body that emits radiation at the same value it absorbs it from outside sources must emit radiation at ever-higher intensities as the wavelength shortens.
The 19th-century understanding of physics reached an impasse around 1900 when it proved inadequate to explain the way light is emitted from objects. Under the classical model, an object in thermal equilibrium emits radiation of a given intensity at all possible wavelengths. As the frequency of the emitted light increases near the ultraviolet end of the spectrum, so do the wavelengths of light that must be emitted to match classical models. Eventually, the model predicts that black bodies should radiate infinite energy at high frequency.
A finite body cannot radiate infinite energy. This seeming paradox was eventually resolved by Max Planck. Planck reasoned that the problem of infinite radiation from a finite source would disappear if atoms were limited in the range of energy they were allowed to emit. He proposed that light could not be emitted arbitrarily, but only in discreet packets, which he called quanta. This was one of the first insights into what would come to be known as quantum physics.