Qubit

A qubit is a quantum system with two distinguished levels, conventionally labeled 0 and 1. Unlike a classical bit, it can be placed in a superposition of those levels, and several qubits together can be entangled. A measurement in the computational basis returns a classical bit and disturbs the state.

Nielsen and Chuang is the textbook account this library uses for that definition. Algorithm pages use it only as the carrier of the model. Shor's algorithm counts gates on qubits. Grover's algorithm uses n qubits to index N = 2^n candidates. Neither theorem is a claim about a particular physical implementation — trapped ions, superconducting circuits, or photons — and a device with a given number of physical qubits is not yet a device that can run those circuits. See fault tolerance.