two-state (or two-level) quantum-mechanical system

A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics.

a unit of quantum information—similar to a classical bit

In quantum computing, a qubit (short for quantum bit) is a unit of quantum information—similar to a classical bit.

a basic unit of quantum information that can take on richer states that extend beyond just 0 and 1

In quantum computing, a qubit is a basic unit of quantum information that can take on richer states that extend beyond just 0 and 1.

one of the smallest units of a quantum computing system

A qubit is one of the smallest units of a quantum computing system.

a unit of quantum information that can simultaneously be both 0 and 1 as well as an infinite number of intermediate states between 0 and 1

A qubit is a unit of quantum information that can simultaneously be both 0 and 1 as well as an infinite number of intermediate states between 0 and 1.

a two-state quantum-mechanical system, such as the polarization of a single photon

A qubit is a two-state quantum-mechanical system, such as the polarization of a single photon: here the two states are vertical polarization and horizontal polarization.

a quantum version of a computer bit, which is the smallest unit of data in a computer

A qubit, in simplest terms, is a quantum version of a computer bit, which is the smallest unit of data in a computer.

a unit of information described by a zero, one or can be a mix of both the states in which case which case becomes a superposition

A qubit is a unit of information described by a zero, one or can be a mix of both the states in which case which case becomes a superposition.

a quantum system that encodes 0 and 1 into two distinct quantum states

Well a qubit is a quantum system that encodes 0 and 1 into two distinct quantum states.