To me a grail watch is one that you usually covet for ‘some time’ but that you will never sell.
Typically grails are watches that are sought after by collectors for long periods. However I will say ‘usually’ covet for some time as I believe there are instances where a watch becomes available that you never thought would, or that you never even knew existed, but instantly generates a level of desire and need similar to that of something coveted for a long period.
However for it to be a true grail it must fall into the category of never to be sold.
See full article at http://sweepinghand.co.uk/what-is-a-grail-watch-and-why/