I always call it "Zero Volts", unless it's an actual connection to physical earth, like a buried ground mat for an LF antenna.
The standard prototypes for kilogram and meter were once stored in Sevres, just outside Paris.
According to more or less unreliable sources, the stables at Sevres also contained the standard horse that was used to define the horse power :-). Unfortunately that horse got old and died and most of the old have converted to (kilo)watts.
According to even more unreliable sources, a universal ground potential is available at a certain grounding rod in Sevres :-) :-).
Hopefully anyone reading this will understand that there are no such thing as an absolute ground potential. In practice, everything is differential and should be treated as such.
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