The Holy Grail is the chalice of the Last Supper in which the wine was given the significance of the blood and life of the Divine.

As it is at the focus of the pouring of Divine life into the world, creating and sustaining it by an act of sacrifice which is both eternal and in time--in eternity the "sod ha tsimtsum," the mystery of the Divine making space, and in time the mystery of the Incarnation and the Cross, so--as that focus--the Grail is (or at least is an ideal image) of that which is the focus of the universe itself as vessel of the Divine, for all the worlds and levels together form a single vessel. In this aspect, to digress for a moment from this rather dense definition, the Grail would appear to be the image that the popular novelist Stephen King is reaching for in his Dark Tower series when he speaks of a mystically fascinating "nexus" or "pylon" around which all possible worlds revolve...and indeed his character Roland may be seen as a modern Grail knight, his depiction is a bit colored perhaps by the conventions of both modern "hard--boiled" realism and "horror" fiction, but completely recognizable finally as a companion of the quest. 

Now, to get back to business, not only is the universe a "vessel" but so is the individual soul, and that is why the Grail is so truly the archetype of the individual's spiritual way. Furthermore the one Grail gathers up the symbolisms of all those other and lesser (because realized only on the level of imagination) cups--the Krater of the Platonists, the Cauldron of Ceridwyn  and so on--so that they are seen as rays of rainbow light from that one prism and heart of light which is the Grail. The Grail is, again, the transformation and restoration (indeed a transformation which goes beyond restoration!) of the world. 

As it is the vessel of the Eucharist, so it is also the source of the Eucharist today, whether served in an ancient Gothic cathedral or in some store-front in the inner city, however in particular I daresay the spirit of the Grail mythos resides in the Eucharist experienced as the Eucharist of the 'Eschaton', of the 'end' (the 'term' of Plotinus), towards which all things rush. 

I believe you will find this a rather complete definition; the "alchemical" approach to the Grail for example simply expresses this same material in terms of alchemical symbolism. If anything, many approaches to the Grail involve a reduction of the vision to a symbolism, finally one of many interchangeable symbolisms, of personal transformation. Well, without wishing to overstress what must be an individual discovery, we counsel against that sort of reduction and in favor of the hard climb to the place where one can see the Grail in its breathtaking universality and Glory.

II.  What is achievement of the Grail?

Now, we have said already that achievement of the Grail involves becoming, and knowing oneself as, a vessel of the Divine and that it is a hard climb to vision. We will say more when we come to make some recommendations for the way, and of course we are aware that there is little use piling on words that are beyond our experience, but it seems appropriate to add one thing here. That is that because the Grail is that which sustains the world and all the "ten thousand things" in existence--by precisely the Divine act of "Tsimtsum', of Sacrifice on all levels--the passing beyond of the vision of the Grail is not that of disappearance into the Divine but that of free and loving adoration.   Again, and this is very important to meditate and work with inwardly, the Grail that precisely which sustains and validates all things, including your existence and mine, and its achievement involves knowing oneself in relation to the Fountain of Life.

This excerpt is from The Quest for the Grail, Lecture Seven See also What is The Grail? You may view the entire document, or go directly to the Archives by selecting these  underlined links.