Decoherence

Decoherence is the loss of a quantum state's coherence because the system becomes entangled with degrees of freedom you are not computing on. Interference that an algorithm was counting on no longer happens. The device is still quantum. It is just quantum jointly with an environment you do not control.

Shor's 1997 factoring paper already treats decoherence, and imprecise gates, as the obstacle to running the long circuit the complexity bound assumes. His 1995 memory code is an early answer aimed at storage errors: a way to recover a logical state if the noise matches the code. That is the start of fault tolerance, not the end of it. NISQ is the name for machines on which decoherence still sets the circuit depth. When an algorithm page says a result assumes an ideal gate array, decoherence is the hypothesis that has been set aside.