A two-state quantum system which is the smallest unit of quantum information. Its two basis states are conventionally labeled |0> and |1> (pronounced: ket 0 and ket 1). A pure qubit state is a linear superposition of those two states. This is significantly different from the state of a classical bit, which can only take value 0 or 1.
A number of Qubits taken together is a qubit register.
It is also possible to have a three-state system, called a qutrit, whose states are conventionally labeled |0>, |1> and |2>.