Android 4.4 (codename KitKat) is API level 19 of the mobile operating system developed by Google.
Google released Android 4.4 (KitKat) on 31 October 2013. The Nexus 5 was the first device to officially ship with it, with Nexus 7 receiving the update soon after.
More Info:
- Official Documentation
- Android 4.4 and Updated Developer Tools
- Getting Your SMS Apps Ready for KitKat (important change for SMS applications)