Touchstone, in its original form, refers to a stone, that was used during medieval times, to test whether a metal composite is actually gold.
In software testing, I find that the emphasis, on what should be used, as the basis for evaluating the test results, gets lost in the catch-all phrase of ‘requirements’. With requirements, the focus is primarily on that which is new and in conception stages as opposed to a 360 degree view of what holistically constitutes as acceptable.
There is a term called test oracle, but I have rarely observed it being used colloquially. It continues to remain in the realm of theory. For something that plays such a central role in the testing effort, it is a shortcoming to not explicitly call out what is the test oracle. In practice, requirements, and that too functional requirements, largely constitute the test oracle.
Touchstone will be used across blog posts on this site to refer to the comprehensive standard or points of reference against which the outcome of the test effort can be compared to determine alignment with desired or expected behavior.