To say that something is 'potential' is to say that it has a particular sort of relation by which the actuality of something is possible for it. It is, in short, a relative term. Because of this, it can actually be applied to very different things as long as the relation is maintained.
Sometimes when we say that something is potential for X, we are saying that it is related to X by a lack of actuality that X's actuality could complete; X's actuality is only a possibility that can happen to it. This is potentiality in the strictest sense; in this sense something is potential precisely insofar as it is lacking in actuality. This potentiality is, so to speak, the capability of the thing that has it to be caused in some way.
However, at other times, we mean instead an actuality that can be the source of the actuality of other things, so that X's actuality is possible insofar as it is related to that source. Thus we might say that the sun has the potential to bring forth life; this bringing forth of life is not the actualization of any lack in the sun but something whose actuality is possible because the sun's superabundant activity is what can actualize it.
These two potentialities are very different, but often confused; recognizing the distinction can clear up many difficulties, not just with respect to discussions of the potential but similar discussions of power and disposition and capability and the like.
