Quantum glossary

Mixed state

A state that is a statistical mixture of pure states, describing classical uncertainty on top of quantum uncertainty.

An equal mixture of |0⟩ and |1⟩ and the superposition (|0⟩+|1⟩)/√2 give the same Z-measurement statistics, but a Hadamard tells them apart: the superposition becomes |0⟩ with certainty, the mixture stays 50/50.

See it for yourself

VisualizationMixed States and the Density Matrix

Density matrix — A matrix description of a quantum state that can also represent uncertainty about which state you have.
Decoherence — The loss of quantum behaviour as a system becomes entangled with its uncontrolled environment.
Superposition — A quantum state that is a weighted combination of basis states, with complex amplitudes.