Final answer:
The WTO principle requiring equal tariff treatment for all member-states when one member-state lowers tariffs for another is known as the most-favored-nation principle.
Step-by-step explanation:
The principle that mandates that when a World Trade Organization (WTO) member-state lowers tariffs on certain kinds of imports from another member-state, all other member-states must receive the same treatment for their goods, is known as the most-favored-nation (MFN) principle. This principle is a fundamental part of the WTO's efforts to ensure non-discriminatory trade practices amongst its members. By lowering barriers to trade, such as tariffs, all WTO member states can benefit from increased market access and economic gains. The WTO provides a platform for its members to negotiate reductions in trade barriers, which can have significant positive impacts on the global economy, as seen in the potential increased economic output from the Doha Round negotiations.