What is the defining feature of a Health Maintenance Organization (HMO)?

An HMO is a managed-care plan that generally requires members to receive care from a defined network of providers (except in emergencies) and to choose a primary care physician (PCP) who coordinates their care and provides referrals to specialists. HMOs typically have lower premiums and out-of-pocket costs but less flexibility, and out-of-network care is usually not covered except in emergencies. This contrasts with a PPO, which offers more provider choice without requiring referrals but at higher cost. Understanding the HMO's network, PCP-gatekeeper, and referral features is core managed-care content on the health insurance exam.