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A Mainframe is a powerful, high-performance computer used primarily by large organizations—such as banks, airlines, government agencies, and major retailers—for mission-critical tasks like mass data processing, high-volume online transaction processing (OLTP), and enterprise resource planning (ERP). Distinguished by their exceptional reliability, availability, serviceability (RAS), and security, modern mainframes are designed to handle billions of transactions daily and manage enormous databases with virtually zero downtime. While their physical size has shrunk from room-sized behemoths to something resembling a large refrigerator, their core strength remains their ability to support thousands of users and applications simultaneously, often running multiple operating systems via virtualization, making them the resilient and trusted "big iron" foundation of the world's commercial and financial infrastructure.