The basic, central, or essential point or feature.
The critical or transitional moment or issue, a turning point.
The hardest point of a climb.
A puzzle or difficulty.
A cross on a coat of arms.
The most important one out of a related group.
The ship occupied by the fleet's commander (usually an admiral); it denotes this by flying his flag.
The ship regarded as most important out of a group, e.g. a nation's navy or company's fleet.
To act as a flagship for.