Doughnut Hole, Cadillac plans, Capitation, Bundling. The health-care debate that has spilled out of Congress and into town halls and living rooms across the U.S. has introduced the average Joe to a baffling array of jargon. But none has had such resonance as the term "public option," even if a majority of Americans still don't know what it means. So here it goes, write this down: In the simplest terms, "public option" refers to a government-run health insurance plan that would compete with private ones and, in the words of President Barack Obama, help "keep insurance companies honest." The likeliest public-option design would be available to less than 5% of Americans. Still, Republicans and private insurers are opposed to a public option, insisting that it represents a federal invasion of the free market.