Android 4.4 KitKat has API level 19.
An Android API level is an integer that identifies the framework APIs and platform behavior available to applications. Developers use it when setting values such as minSdk, targetSdk, and compileSdk in an Android project. API level 19 corresponds to Android 4.4, whose consumer codename was KitKat.
Android 4.4 was released in October 2013. It focused heavily on improving performance and reducing memory use, allowing the system to run on devices with as little as 512 MB of RAM. It also introduced platform features such as immersive full-screen mode, printing support, and improved WebView behavior.
A common mix-up is Android 4.4W, the wearable edition of KitKat. That release used API level 20, while standard Android 4.4 KitKat used API level 19. Android 5.0 Lollipop then advanced to API level 21.