In quantum computing, a qubit () or quantum bit is a basic storage/symbol, into which a bit of quantum information is stored/encoded-the quantum version of the classic binary bit physically realized with a two-state device. A storage is preferred terminology in computing whereas a symbol is preferred terminology in digital communication.

However, it does not mean that a qubit is a basic unit of quantum information, because, a classical bit has two meanings: one as a unit of information another as a binary storage or a binary symbol into which a bit, as a unit, of information can be stored or encoded. A bit or a qubit "physically realized with a two-state device" is a physical entity, a physical storage/symbol, and is not an abstract concept of a unit of information. Just as a unit of quantum energy is not quJ but J (Joule), a unit of quantum information or quantum entropy is not a qubit but a bit.

Both in classical and quantum systems, a storage/symbol, in general, can have more than two states. However, a qubit only uses two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems, into which, only a single bit of information can be stored/encoded, from which, at most a single bit of information can be retrieved/decoded. 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.

That a qubit can be in a coherent superposition of both states simultaneously is not a property that is fundamental to quantum mechanics or quantum computing, because both photon and classical polarization states are represented by points on a Poincaré sphere or Bloch sphere ignoring amplitudes and global phases and can be superpositioned with complex number coefficients. Superposition of electron spin is quantum not because superposition is quantum but because half integer spin of an electron is quantum.

Quantum computing is not classical but quantum only when it involves entangled states with multiple qubits.