Noisy intermediate-scale quantum era
In the noisy intermediate-scale quantum (NISQ) era the leading quantum processors contain about 50 to a few hundred qubits, but are not advanced enough to reach fault-tolerance nor large enough to profit sustainably from quantum supremacy. The term was coined by John Preskill in 2018. It is used to describe the current state of the art in the fabrication of quantum processors.

The term 'noisy' refers to the fact that quantum processors are very sensitive to the environment and may lose their quantum state due to quantum decoherence. In the NISQ era, the quantum processors are not sophisticated enough to continuously implement quantum error correction. The term 'intermediate-scale' refers to the quantum volume related to the not-so-large number of qubits and moderate gate fidelity.

Algorithms 
The term NISQ algorithms refers to algorithms designed for quantum processors in the NISQ era. For example, the variational quantum eigensolver (VQE) or the quantum approximate optimization algorithm (QAOA), are hybrid algorithms that use NISQ devices but reduce the calculation load by implementing some parts of the algorithm in usual classical processors. These algorithms have been proven to recover known results in quantum chemistry and some applications have been suggested in physics, material science, data science, cryptography, biology and finance.

Usually, NISQ algorithms require error mitigation techniques to recover useful data.

Beyond-NISQ era 
The creation of a computer with tens of thousands of qubits and enough error correction would eventually end the NISQ era. These beyond NISQ devices would be able, for example, to implement Shor's algorithm, for very large numbers and break RSA encryption.

References

External links 
 John Preskill lecture on NISQ era

Computer architecture statements
History of computing hardware
Quantum computing
Quantum information science
Computational complexity theory