A quantum state is the condition a quantum mechanical system is in. The state is usually described by a wave function. It is different from classical states of systems in that the quantum system has separate, discrete levels, whereas the states of a classical system are continuous. This means that a particle can move from one quantum state to another without ever being in a state in between. The discrete steps are known as quantization.

A quantum leap or jump is the transition between quantum states. Although colloquial usage varies, quantum leaps are almost always very small--only a fraction of the system's total energy. These quantum state changes are responsible for common phenomena like light emission from atoms. They can occur spontaneously (as in radioactive decay) or can be stimulated to occur (as in lasers).

Each quantum mechanical system has special stationary states known as eigenstates...