"Bytecodes" is a name given to the instructions of a VirtualMachine, so called because the opcodes are 8 bits (one byte) in length, and instructions vary from one to several bytes in length. Unless otherwise specified, the term "bytecodes" refers to those of the Java Virtual Machine (JVM), but the term can also apply to the executable files of other virtual machines, e.g. Microsoft .NET assemblies. Java Bytecodes are found in Java executable ( .class ) files.

Bytecode instructions perform a similar job to native machine instructions, except that bytecodes are machine independent. They are compact (1.8 bytes per average instruction is claimed for Java bytecodes, though this neglects overheads such as the constant pool). Some bytecode instructions are quite high level, e.g. dynamically invoking a virtual method by name.

-- MikeVanEmmerik - 01 Dec 2001