touchstone

1. (figurative, by extension) A standard of comparison or evaluation.Examples: "The foregoing doctrine affords us also a touchstone for the trial of spirits."; ""Expectation of life" is an elastic term: the touchstone here is rather the aggregate amount of work that the locomotive does during its working life."; "Although there has not been very much direct treatment of the Holocaust in analytic moral philosophy, the Nazis appear again and again as examples or touchstones in the construction of moral theories."broadlyfiguratively