The balance of trade
The difference between the value of the imports and exports that a nation makes.
This expression is of uncertain origin and dates back to at least the 17th century. From Sir Josiah Child's A New Discourse on Trade, published 1668:
"The Ballance of Trade is to be taken by a strict scrutiny of what proportion the value of the Commodities exported out of this Kingdom bear, to those imported."
Given that this is a definition of the term and is the earliest known reference, it is quite likely that Child coined it himself.