Qubit
The quantum computer's smallest unit — can be zero, one, or a combination of both right up until it is measured.
Physically a qubit can be anything with two distinguishable quantum states: an electron's spin, a photon's polarisation, the energy levels of a superconducting circuit. The term was coined by Benjamin Schumacher in 1995.
The difference from a bit is not that a qubit stores more information — measuring yields exactly one bit, and the superposition is gone. The difference is what can be done before measurement. The state is described by amplitudes that can be negative, and it is that possibility of cancellation which gives quantum algorithms their power. A common misconception is that n qubits hold 2ⁿ values at once; they evolve according to 2ⁿ amplitudes, but only one value can be read out.