classical limit
The classical limit is the regime in which the predictions of quantum mechanics blur smoothly into the familiar physics of everyday objects. Quantum mechanics is believed to be the deeper theory, so it must reproduce the well-tested results of Newtonian mechanics whenever those results are accurate. The classical limit is where that recovery happens: large, heavy, warm, complicated systems behaving just as classical physics says they should.
Several things conspire to bring it about. For massive objects the de Broglie wavelength is fantastically tiny, so wave effects like interference and the spread required by uncertainty become utterly negligible at human scales. The graininess set by Planck's constant ℏ is so small compared with everyday actions that the quantization of energy and the uncertainty floor simply disappear into the rounding. In this sense the classical world is what quantum mechanics looks like when ℏ is, for practical purposes, vanishingly small.
Decoherence completes the story. A large object is constantly buffeted by its environment — stray photons, air molecules, thermal vibrations — which rapidly destroys the delicate phase relationships that make quantum superpositions observable. What remains behaves like a classical probability mixture with definite-looking outcomes. The classical limit is therefore not a sharp boundary but a gradual fading: quantum strangeness is always present in principle, just immeasurably suppressed for big, entangled-with-everything objects.
When Planck's constant is tiny next to the action involved, quantum effects fade into classical physics.
The classical limit is not a place where quantum mechanics stops being true; it is where its predictions become indistinguishable from classical ones. Decoherence explains the absence of large-scale superpositions without requiring any physical 'collapse' to be added by hand.