The "Big Four" refers to the four largest accounting firms in the world: Deloitte, Ernst & Young (EY), PricewaterhouseCoopers (PwC), and Klynveld Peat Marwick Goerdeler (KPMG). They dominate the accounting and auditing industry, providing services like financial statement audits, tax advice, and consulting to a vast number of companies globally, including most of the world's largest corporations. Their size and influence make them key players in the global financial system.