PCI stands for Payment Card Industry, but usually means one of the following:
The Payment Card Industry Security Standards Council. This is an industry body made up of organizations like Visa, MasterCard, American Express and Discover. The Council is how these companies cooperate to agree upon a single, common security standard that Merchants are required to meet.
The actual security standard put together by the Council described in the first definition above. The full name for this standard is the Payment card Industry Data Security Standard (PCI DSS). Merchants must meet this set of security requirements if their business accepts, transmits or processes customer payment cards, such as credit cards or debit cards.