L33t or its various other titles* is a derivation of English but I have no idea what term should be used to describe it. Other examples of these things would include lolspeak and the massive alterations involved in commonplace texting.
Wikipedia suggests that l33t is an alternative "alphabet" but that only covers the letter/number conversions. "Alphabet" doesn't address the lingo and structure (*cough*) that l33t employs.
Perhaps this question is too far outside of the scope of this site. L33t itself suggests a culture of its own but no one over there can agree on anything and any popular lingo today will be forgotten or replaced by tomorrow. Its worthiness is totally debatable but I still want to know what to call it.
* Included here for future searches: leet, leetspeak, l33tspeak, l33tsp34k, 1337, 13375p34k