[wrs]

The JVM is a native-code application (i.e., running on the processor directly) that executes the bytecode by interpreting it (starts fast, but runs slowly) and/or converting it into native code and running that code on the processor directly (delayed by the conversion step, but then runs fast). The latter step, translating bytecode to native code, is known as the just-in-time (JIT) compiler.

So somewhat confusingly, Java typically involves two compilers: one from source code to bytecode, the other from bytecode to native code. By nature, the first is processor-independent, the second is processor-dependent. The second happens at runtime on the target machine — thus, “write once, run anywhere”.