Noun

1. A fundamental rule or principle.
   • 1859, Charles Darwin, On the Origin of Species, ch. 10:

     It is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties.

2. The principle that one should treat other people in the manner in which one would want to be treated by them.
   • 1818, Sir Walter Scott, Rob Roy, ch. 2:

     "Mr. Francis seems to understand the fundamental principle of all moral accounting, the great ethic rule of three. Let A do to B, as he would have B do to him; the product will give the rule of conduct required." My father smiled at this reduction of the golden rule to arithmetical form.