I can not find a coherent and applicable definition of the platform libraries in Java. I am really a very novice to programming and as I can see platform libraries are thought as something widely known and all sources are careless to give a definition of it. So what are platform libraries in Java? I would like to see definition and if possible some examples of them.
The "platform libraries" (also referred to as the "Java standard library") are what is included with the Java runtime, in other words: the classes, interfaces, etc that you can use without explicitly having to add (external) libraries to your class path.
It is not carelessly defined, it is governed by Java specifications like the Java SE 8 Platform Umbrella JSR (337). However people are so used to this, that they do not feel the need explicitly specify it further when mentioned.
An overview can be found in the Java 8 documentation and javadoc.