A business practice that involves the consolidation of all companies offering a service or product to reduce competition.
Horizontal integration is a business practice that involves the consolidation of all companies offering a service or product (say, tires) to reduce competition. If Will Stinson Tires buys out as many other tire companies as it can to merge resources, it is streamlining through horizontal integration.
Horizontal integration allowed some businesses to become the only supplier of a certain product. This monopolization allowed them to set the price however high they pleased. This, with vertical integration, is a prime example of the kind of aggressive (and questionably ethical) pursuit of efficiency that made men like Rockefeller so rich.