A standard, against which all others of the same category are measured. Usage: silly.
The notion is that one of whatever it is has actually been cast in platinum-iridium alloy and placed in the vault beside the Standard Kilogram at the International Bureau of Weights and Measures near Paris, as the bar defining the standard metre once was.
"This garbage collection algorithm has been tested against the platinum-iridium cons cell in Paris."
The normal standards of the metrical system are made of platinum-iridium on account of its known immutabilty.
The last three values are those obtained by extrapolation with platinum-rhodium and platinum-iridium couples.