Basic Definition:

A single-payer healthcare system is where a single entity is responsible for collecting the funds that pay for healthcare on behalf of an entire population. In contrast, a multi-payer system allows multiple entities (e.g., insurance companies) to collect and pay for those services.

Expanded Explanation:

A Single-Payer system, AKA Medicare for All (M4A), is a publicly-funded healthcare financing system that pools all health care dollars into a single government-guaranteed trust. The public trust pays the bills, but private providers deliver health care. 

A single-payer health care system is the most cost-effective way to guarantee the same high standard of care to the population without financial barriers. Countries with single-payer systems spend less while providing comprehensive, high-quality health services to all residents, resulting in better population health outcomes. Single Payer is patient-centered.

In contrast, the United States currently has a multi-payer health care system, involving thousands of health plans within a complex, fragmented, expensive, inequitable, inefficient, and profit-driven landscape.  A multi-payer system allows multiple for-profit entities to establish tiers of care and the ability to determine, delay or deny who gets what care. This uniquely American health insurance system is a business designed to extract profit by maximizing cost and minimizing care, to reward owners and shareholders with ever-increasing financial gain.

Multi-payer is profit-centered.