In standard Java, the accessible classes are the classes present in the classpath or whose containing .jar file is in the classpath.
By accessible, I mean that you can refer to these classes in your source code. By example, you can type
System.out.println("hello");
because the System class (or rather its containing .jar) is in the classpath.
Is there a notion of classpath in Android ?
What are the accessible classes ?
In particular, how can I make a class in an apk accessible to classes in other apks ?
I dont think this is possible. You can’t directly call methods located in other apks. However it is possible to communicate with other APK’s by using Intents.