PPO is a Three letter abbreviation for:
1-Preferred provider organization
2-Public Policy Office
3-Prisons and Probation Ombudsman
4-Poly(p-phenylene oxide)
1)-Preferred provider organization :-
In health insurance, a preferred provider organization (or "PPO") is a managed care organization of medical doctors, hospitals, and other health care providers who have covenanted with an insurer or a third-party administrator to provide reduced rates to the insurer’s or administrator’s clients.
Concept :-
The idea of a preferred provider organization is that the providers will provide the insured members of the group a substantial discount below their regularly-charged rates. This will be mutually beneficial in theory, as the insurer will be billed at a reduced rate when its insured utilize the services of the "preferred" provider and the provider will see an increase in its business as almost all insureds in the organization will use only providers who are members. Even the insured should benefit, as lower costs to the insurer should result in lower rates of increase in premiums.
PPO V/S HMO :-
Unlike health maintenance organizations, in which insureds who do not utilize members of the organization as their health care providers receive little or no benefit, PPO members will be reimbursed for utilization of non-preferred providers, albeit at a reduced rate which may include higher deductibles, co-payments, lower reimbursement percentages, or a combination of the above.
More at (http://www.reference.com/browse/wiki/Preferred_provider_organization)
2)-Public Policy Office :- of APA (American Psychological Association)
(APA’s Public Policy Office)
The Public Policy Office of the American Psychological Association represents the largest and most visible national presence advocating for psychology. APA advises congressional decision-makers on a wide range of legislative and regulatory issues.
MORE AT (http://www.apa.org/ppo/)
3)-Prisons and Probation Ombudsman :- (For England & Wales website)
The Prisons and Probation Ombudsman is appointed by the Home Secretary and investigates complaints from prisoners and those subject to probation supervision, or those upon whom reports have been written. The Ombudsman is completely independent of both the Prison Service and the National Probation Service (NPS). The current Ombudsman is Stephen Shaw, and a team of deputies, assistants, investigators and other staff supports him.
The Ombudsman is also responsible for investigating all deaths of prisoners and residents of probation hostels and immigration detention accommodation.
More at (http://www.ppo.gov.uk/)
4)-Poly(p-phenylene oxide) :-
Poly(p-phenylene oxide) (PPO) is a conducting polymer of the semi-flexible rod polymer family.
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So, it was quite an open-ended question.I hope this answer may have helped.