What is a qubit?
Qubit
basic unit of quantum information—the quantum version of the classical binary bit physically realized with a two-state device.  A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics
In quantum computing, a qubit(/ˈkjuːbɪt/)or quantum bit(sometimes qbit) is the basic unit of quantum information—the quantum version of the classical binary bit physically realized with a two-state device.  A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics.  Examples include: the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superpositionof both states/levels  simultaneously, a property which is fundamental to quantum mechanics and quantum computing.