It originally was a system by which countries set the value of their currency to an specific amount of gold. For example $100 might equal an oz of gold. So that it was easy to understand the value of your currency to another country.
In common vernacular, it simply means “the best” of something of which all derivatives strive to be. For example the gold standard vacuum cleaner would be a brand or model so widely accepted as the best that all companies strive to build their vacuum to meet or beat it.
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