What term is defined as the idea of each country concentrating on making those goods it can produce most efficiently and buying from other nations those goods it cannot make as efficientl?

The term you're referring to is "comparative advantage." It describes the economic principle that countries should specialize in producing goods for which they have the lowest opportunity cost and trade for others, thereby increasing overall efficiency and benefiting all parties involved in the exchange. This approach fosters international trade and allows nations to allocate resources more effectively.