What is Java Bytecode ?

• When a Java program source is compiled, the output of the Java compiler is called bytecode. 
• Many language generate executable code after compilation. 
• In Java the output is converted to a special type of file which is called the bytecode.
• Bytecode is a highly optimized set of instructions designed to be executed by the Java run-time system,which is called the Java Virtual Machine (JVM). 
• Bytecode makes it much easier to run a program in  a wide variety of environments because only the JVM needs to be implemented for each  platform.

Java: A Beginner's Guide, Seventh Edition