Application Protocol Data Unit (APDU) definition in Computer Security terms:
Definition(s): A part of the application layer in the Open Systems Interconnection Reference model that is used for communication between two separate device’s applications. In the context of smart cards, an APDU is the communication unit between a smart card reader and a smart card. The structure of the APDU is defined by ISO 7816-4.
Source(s): NIST SP 800-166
reference: CSRC Glossary