PCI DSS stands for Payment Card Industry Data Security Standard, which is a set of security requirements developed by the major credit card companies to protect sensitive cardholder data. PCI DSS applies to any organization that accepts, processes, stores, or transmits cardholder data, and includes requirements for network security, access control, data protection, and monitoring. Compliance with PCI DSS is mandatory for all merchants that accept credit card payments, and failure to comply can result in fines, legal liabilities, and damage to a merchant's reputation.