A unit or department within an organization that standardizes project management processes and helps with the sharing of resources, best practices, tools and techniques across the company.
Depending on its maturity, the PMO can provide simple support services (administrative support & sharing of best practices), expertise on key project performance elements and metrics (e.g. estimations, scheduling, risk management, quality assurance…), act as consultant and advisor to project managers (including with issues relating to HR and feedback), or, at its most advanced, play a key role in strategic and performance-related decision-making.
Related articles: building an effective enterprise portfolio management process