quantum mechanics - What is a coherent state?

In quantum mechanics, what exactly is a coherent state, and how does it differ from other states?

Answer

A coherent state is a state that is not an energy eigenstate, so the observables vary in time, but the uncertainty of the canonical conjugates (position and momentum) stays constant - unlike, for instance, in an ordinary wave packet moving in free space: this "floats away", the position uncertainty gets ever bigger and bigger.