From Longman Dictionary of Contemporary EnglishHoly GrailˌHoly ˈGrail noun 1 TRY TO DO OR GET something[singular] something that people want very much, but which is very difficult or impossible to achieveHoly Grail of Nuclear fusion is the Holy Grail of energy production.2 → the Holy Grail
Examples from the Corpus
Holy Grail• That was my Holy Grail, and there could be no real life for me until I found it.• A vaccine for malaria has become something of a scientific holy grail.• The plot proper, however, is the Big One - a quest for the Holy Grail.• Another generation of Labour leaders, in the 1960s, were still left to search for the Holy Grail of economic planning.• We may remember here that the Holy Grail is sometimes considered to be a stone, as well as a vessel.• Total immunity from colds once seemed to be the unattainable Holy Grail of clinical viral science. 