[eatonphil]

Most terms in language implementation are fuzzy. But just in time compilation most often refers to switching from interpreting bytecode to (generating machine code and running that) generated machine code in specific spots after having analyzed the currently running bytecode for a while.

"Classical" (again, every term is fuzzy) JIT compilers either do this machine code compilation after seeing a good candidate _entire function_ or a good candidate _section of code within a function_. Good candidates are often areas of code that are executed a large number of times and with consistent internals (e.g. iterating from 0 to 10000 with variables inside that have provably fixed types).

But there are infinite variations of JIT compilation.

In any case, CPython doesn't do that switching from bytecode to generated machine code. Pypy does do that. As does V8 and the JVM and so on.