A nickname for the Patient Protection and Affordable Care Act, a law signed by President Barack Obama in 2010 that set in motion sweeping changes to the U.S. health care system, including the requirement that private health insurance companies cover people with pre-existing conditions, copay-free preventive care including birth control, and a requirement that young people can stay on their parent or guardian’s insurance until they’re 26 years old. The law is frequently called ACA, for short.