# APDU

> Application Protocol Data Unit -- the communication unit between a smart card and a reader defined by ISO 7816-4.

**Also known as**: Application Protocol Data Unit

## Related Terms
- [ATR](https://smartcardfyi.com/glossary/atr/)
- [T=0](https://smartcardfyi.com/glossary/t0/)
- [T=1](https://smartcardfyi.com/glossary/t1/)
- [SWP](https://smartcardfyi.com/glossary/swp/)
- [APDU Response Codes](https://smartcardfyi.com/glossary/apdu-response-codes/)
- [Secure Messaging](https://smartcardfyi.com/glossary/secure-messaging/)
- [ISO 7816](https://smartcardfyi.com/glossary/iso-7816/)
- [Card Testing](https://smartcardfyi.com/glossary/card-testing/)