Definition of a single payer, as per the Congressional Budget Office:

"Although single-payer systems can have a variety of different features and have been defined in many ways, health care systems are typically considered single-payer systems if they have these four key features:

• The government entity (or government-contracted entity) operating the public health plan is responsible for most operational functions of the plan, such as defining the eligible population, specifying the covered services, collecting the resources needed for the plan, and paying providers for covered services;
• The eligible population is required to contribute toward financing the system;
• The receipts and expenditures associated with the plan appear in the government’s budget; and
• Private insurance, if allowed, generally plays a relatively small role and supplements the coverage provided under the public plan"