Question

in the definition of radiative lifetime below. what do they mean by depopulation. do they mean for 1 given energy state the amount of time until the electron gives off enough photons to be at the lowest state? or just the electron moves down 1 state? or the lifetime of a collection of mass unti all electrons in that mass have reached the ground state?

The radiative lifetime of an excited electronic state e.g. in a laser gain medium is the lifetime which would be obtained if radiative decay via the unavoidable spontaneous emission were the only mechanism for depopulating this state. It is given by the equation

Answer 1

In the simplest terms the depopulation of an energy state can be understood as the decrease in the population of the particles present in that state as the particles loose their enrgy by emmiting spontaneous emmision.

consider this case to make it more clear, imagine that you give certain amout of energy to an atom. Now the elctrons of this atom will be excited to various states. But after certain amount of time they will start to come down again. In any one of the shells if their were 5 electrons after excitation and only 2 are left after some time then these three electrons that have gone down constitutes what is known as depopulation.   

So your second point, that "or just the electron moves down 1 state" , is more closer to the truth. In reality WHERE the electron moves down to doesn't matter, weather by 1 state or 2. What matters in depopulation is that it leaves the CURRENT state. kindly review my answer and hit the LIKE button if my answer makes the point clear.