UPI, or the Unified Payments Interface, is an instant real-time payment system in India that allows users to transfer funds between bank accounts using a mobile app. It's a platform developed by the National Payments Corporation of India (NPCI). UPI facilitates various types of transactions, including peer-to-peer (P2P), person-to-merchant (P2M), and person-to-merchant (P2PM) for small merchants and unorganized retail sectors.