Define 'preferred provider organization' (PPO).

A preferred provider organization (PPO) is defined primarily by its structure, which allows members greater flexibility in choosing healthcare providers. In a PPO plan, members are encouraged to use a network of preferred providers, which facilitates easier access to healthcare services at reduced costs. This flexibility means that individuals enrolled in a PPO can see specialists without needing a referral and are not restricted to a specific network of doctors, though they may pay lower copays or deductibles when using in-network providers. This structure contrasts with other types of health plans, such as health maintenance organizations (HMOs), which typically require members to choose a primary care physician and get referrals for specialist services.

The other options describe different aspects of insurance but do not capture the essence of what a PPO offers. Emergency medical services, basic insurance plans with limited choices, and programs for subsidizing out-of-pocket expenses represent different types of health plans or insurance features that do not specifically highlight the core characteristics of a PPO's flexibility and provider options.