A "grandfathered" health insurance plan refers to a health insurance policy that was in place before the Affordable Care Act (ACA or “Obamacare”) was signed into law on March 23, 2010. These plans are exempt from some of the ACA’s requirements, meaning they do not have to adhere to all of the new rules and protections established under the ACA. Below are some key characteristics of grandfathered plans:

1. Exemptions from ACA Rules: These plans are not required to cover certain benefits mandated by the ACA, such as preventive care without cost-sharing, and they may have higher out-of-pocket costs compared to ACA-compliant plans.

2. Maintenance of Status: To maintain their grandfathered status, plans must not make significant changes that reduce benefits or increase costs to consumers. If a plan makes too many changes, it loses its grandfathered status and must comply with all ACA requirements.

3. Consumer Protections: While grandfathered plans are exempt from some ACA rules, they must still comply with certain protections, such as the prohibition on lifetime limits on essential health benefits and the requirement to allow young adults to stay on their parents' plan until age 26.

4. Limited Availability: Grandfathered plans are gradually disappearing, as insurers and employers adjust benefits or change plans, causing them to lose their grandfathered status.

In summary, a grandfathered health insurance plan is one that existed before the ACA and is exempt from certain ACA rules, as long as it does not undergo significant changes.