The set of characteristics describing the condition a quantum mechanical system is in. It can be described by a wave function or a complete set of quantum numbers (energy, angular momentum, spin, etc), although, when observed, the system is forced into a specific stationary "eigenstate". If a particle within a quantum system (such as an electron within an atom) moves from one quantum state to another, it does so instantaneously and in discontinuous steps (known as quantum leaps or jumps) without ever being in a state in between.