ultraviolet catastrophe
The ultraviolet catastrophe is the name for a spectacular failure of classical physics applied to blackbody radiation. Treating the light in a hot cavity as a collection of standing electromagnetic waves and giving each one an equal share of thermal energy — a rule that works beautifully for gases — leads to the Rayleigh–Jeans formula. For long, red wavelengths it matches experiment nicely.
But there is no end to how short a wavelength a wave can have, so there are endlessly many of these short-wavelength modes. With each one carrying the same fixed dollop of energy, the formula predicts that the radiated power keeps rising without limit toward the violet and ultraviolet end. Add it all up and a warm object should blaze out an infinite amount of energy — and your fireplace should incinerate the room.
Nature obviously does nothing of the kind; real spectra turn over and fall to zero at short wavelengths. The word 'catastrophe' captures how badly classical reasoning broke, not a real disaster. Planck escaped it by assuming energy is exchanged only in discrete packets, which mathematically choked off the high-frequency modes. That single, reluctant assumption is the doorway to quantum theory.
Classical theory says energy soars without limit toward short wavelengths — which never happens.
The name is mostly historical drama: it was coined later, and Planck was actually motivated more by curve-fitting the whole spectrum than by avoiding an infinity. Still, it captures the depth of the classical breakdown.