Distance from the centre of a graph.
Example
In this graph, because any vertex can be connected to any other by a chain of two edges, they are all centres of the graph and their distance is 2. Therefore, the radius of this graph is 2.
In this graph, because any vertex can be connected to any other by a chain of two edges, they are all centres of the graph and their distance is 2. Therefore, the radius of this graph is 2.