A software engineering approach in which teams keep producing software in short cycles and ensure that the software can be released to production at any time.
Continuous Deployment (CD) is a continuous automation process where deployment of software to production systems is fully automated. This ensures that fixes and new features can be brought to production quickly.
This usually includes having a good set of automated tests to automatically verify the fitness of the software for production use.
See the article Continuous Delivery on Wikipedia for more information.