Means the Zonę Of Potential Agreement, that is such an agreement which satisfies both parties.
Border price
In other words, it is a set of mutually determined and agreed conditions, which are potentially satisfying for both parties.
The border price of both parties determines the border of the ZOPA. An example is a buyer of a car, who is ready to pay £2000 for it and a seller which wants to sell it for at least £1500.
ZOPA