Hardiness Zones or Climate Zones refer to the average cold temperature of an area. In the U.S., the U.S.D.A. creates Hardiness Zone maps to help gardeners to know what plants will likely survive the winter in their area. The Western Garden book uses a different system but most plants are labeled according to the U.S.D.A. Map. This map was recently updated so your zone may have changed.
Find Zones for other parts of the World. (Scroll down to Hardiness Zones/Climate Zones)