A crux in climbing, mountaineering and high mountain touring is the most difficult section of a route, or the place where the greatest danger exists. In sport climbing and bouldering the most challenging point is also called the crux. In describing a climbing route using a topo, cruces (or cruxes) are usually shown with a key symbol.
The grade of a climbing route is based on the difficulty of the crux. That means the rest of the route can be considerably easier. In addition a route may comprise several cruces. There are also routes, however, that have a very consistent level of difficulty with no sections that stand out as harder than the rest.