What is a Java Runtime Error ??

Answer 1

Runtime error occurs when the program is running ie after the execution begins. These include errors like logical error or when you try to divide by zero or if you leave any object undefined and many such instances.

Answer 2

There are runtime exceptions and errors in Java but both implement the Throwable interface. Any error caused during the execution of the program will be counted as a runtime error.
By the way divide by zero is an Arithmetic Exception and not an error in strict terms.
Below link has the list of all Exceptions and errors in Java.
https://programming.guide/java/list-of-java-exceptions.html