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.
A central cohesive source of stability and security; a person or thing that is critical to a system or organisation.
A pin inserted through holes at the end of an axle or shaft, so as to secure a wheel or shaft-mounted device.
To adopt as, or serve as, a central cohesive source of stability and security.