This is not essentially a religious subject, for it has many non religious applications. An atheist or agnostic can fast just as can a Jew or Muslim, and a baptist as much as a Catholic. Fasting can be used as protest, penance, respect for the deeds of others, or aiding in establishing greater self control, and less focus on the self.
So what exactly is fasting? Fasting is the abstaining of a good (A thing that exists in this context, not a piece of merchandise). That is, very simply, its essence. We fast often throughout our lives, whether voluntarily, or by necessity. I think I'll focus on the first: the voluntary abstaining of a good.
Now, we run into the need for some more clarification. Let's say I hate oranges, so I decide to abstain from them. This would be fasting, but would not be hard at all. Therefore, right fasting is a abstaining from a desired good. for instance if I like running a computer (which I do) and I decided to give up running the computer for a week. That would fit it.
But even so, there is still further statement needed. Let's say I decide to abstain from the computer for a week because I killed a moth. That would be... Pointless (unless you believe killing a moth is an intrinsically evil thing). Thus, fasting is the abstaining from a desired good for a just cause.
But let's say I decided, for a good cause, to abstain from food for the rest of my life. Would that be good fasting? No, it would be suicide. So fasting is reasonably abstaining from a desired good for a just cause.
Now, there is still one more thing. I'm sure most everyone knows the term 'Pharisee' and many know the biblical stories of the pharisees, and how they would fast and whatnot not only for a good cause, but to show everyone how holy they were. That is most certainly not true fasting, so the final definition of fasting is reasonably and humbly abstaining from a desired good for a just cause.
Now, in order to retain the the religious neutrality, I will not seek to define what is a just cause. I'll just throw this out for everyone to interpret this their own way.
I hope you will think of this in the light of various fastings being proposed, and enter into what ones you do with a good perspective. I also hope that this entry in no way violates the BZP rules on such matters (I do not think it does) and if it does, am willing to have it permanently drafted.