Final answer:
The stationary state refers to an object or system that does not move and remains in a fixed position, with wave functions that have a probability density function that does not vary in time.
Step-by-step explanation:
The stationary state, as Mill would understand it, refers to an object or system that does not move and remains in a fixed position. In physics, a stationary state is a state of definite energy, also known as a standing wave state. These wave functions have probability density functions that do not vary in time, meaning their position remains unchanged.
For example, in the context of quantum mechanics, stationary states are solutions to Schrödinger's equation. These states have fixed energy levels and do not evolve with time, like the ground state of a quantum oscillator.
In summary, a stationary state, as Mill would understand it, refers to an object or system that remains stationary and does not undergo any positional changes.