Why is it called a triangle?

The answer

It is called a triangle because the three main stretches of the system, Europe to Africa, Africa to the Americas and the Americas back to Europe, form a triangle when drawn on a map. The name is a historian's teaching aid, not a term people used at the time. Many real voyages followed other patterns, and trade within Africa and within the Americas mattered as well, so a good lesson asks what the triangle leaves out.