On the Smith chart shown in Figure E9-3, what is the name for the large outer circle on which the reactance arcs terminate?

On the Smith chart in Figure E9-3, the reactance arcs terminate at the large outer circle called the reactance axis.
Amateur Extra · FCC Element 4 · answer reveals automatically

On the Smith chart in Figure E9-3, the reactance arcs terminate at the large outer circle called the reactance axis.