If you search "Java API" on the internet, you will find an online description of all the packages included in JDK. Some package names start with java. They are traditionally called core Java packages, while those that start with javax used to be called extensions. This was done so probably because the extensions were thought to be optional and maybe even released independently of JDK. There was also an attempt to promote former extension libraries to become a core package, but that would require the package's name to change from Java to Javax, which would have broken the already existing applications. So, the idea was abandoned, the extensions became as standard part of JDK as the core, and the distinction between core and extensions gradually disappeared.
That is why if you look at the official Java API on the Oracle website you will see...