JITs.

I mentioned in the last lecture tools that will translate JVM bytecode into
native code.  Here

  http://www.sourcemod.net/devlog/?p=76
  http://www.sourcemod.net/devlog/?p=78
  http://www.sourcemod.net/devlog/?p=80
  http://www.sourcemod.net/devlog/?p=81

is a series of four blog posts about building a simple example of such a tool.
"JIT" stands for "just-in-time" and is usually used as an adjective, as in "a
jit compiler" or "jit execution".  A jit compiler is one that selectively
translates bytecode into native code; usually code that's frequently executed,
such as often-called methods, is selected.  Jit translation showed up in
smalltalk systems in the 80s and was brought to its current state by the self
system in the 90s.  Most modern jvms use jit translation.

The posts' author, in addition to using "jit" as a noun, also seems to be using
it to mean a typical compiler that translates a high-level language (bytecode
here) into a low-level language (native code).  Even so, it's nice, simple
introduction to what's involved in translating and why you might want to do it.