1.
The ~ or the Holy ~ is the cup that was used by Jesus Christ at the Last Supper. In medieval times, many people tried to find the ~ without success.
N-PROPER
2.
If you describe something as a grail or a holy grail, you mean that someone is trying very hard to obtain or achieve it.
The discovery is being hailed as The Holy ~ of astronomy.
N-SING: oft the N of n