So in the US, there are two government-funded insurance providers, Medicare – which is meant for Americans of 65 or older, or those with severe difficulties – and Medicaid – which is aimed at providing help for extremely low-income people. Obamacare had provisions for expanding eligibility for Medicaid, that is to say increasing the amount of people who qualify for it, among other provisions. But it doesn’t provide insurance for *everyone*. If you make more money than the threshold, you don’t qualify for Medicare.
Single-payer healthcare is a system by which healthcare costs for for *all* residents are covered by a single public entity. Rather than only some people being eligible for Medicare or Medicaid at any given moment, the entire country would be. The details of the various systems that exist in other countries vary, but that’s the major difference.
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