A puzzle or difficulty.
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 cross on a coat of arms.
A problem that is challenging to solve.
Used other than figuratively or idiomatically: see hard, nut, crack.
A situation, person, group, etc. which is difficult to overcome or deal with.
A place, opportunity, etc. to which it is difficult to gain entry.
An amount that is difficult to finance.