Leet (also called 1337, leetspeak, leetspeek, l33t5p34k, 133t, or l33t) is derived from the word 'elite' and is defined by Wikipedia as a 'form of written slang or street talk for the information highway', which had a twofold purpose: 'to create group identity and to obscure meaning from outsiders'. Used as a way of getting past word filters on BBSs and forums, so that they could discuss topics that were banned or censored, like software piracy, hacks and cracks, it is a written anti-language, a way of communicating within a language that excludes outsiders.
MAK Halliday, the linguist who created the term, suggests that anti-languages have this dual function, that they have a need for over-lexicalisation in order to restrict outsiders' comprehension, an idea embodied in the very name Leet, and to generate a new reality through contrast with the language of the mainstream. Instead of over-lexicalisation, there are increasing degrees of masking in Leetspeak through more and more complex substitutes, including some that often bear little resemblance to the original letter they represent. It has now lost much of its original purpose as it has become too well known, and is often used ironically- '31337 hax0r' (elite hacker) is a phrase used sarcistically in many places.
Most anti-languages are spoken, though early Christians used symbols like the fish as a code, Leet shows that the same concepts can be seen in a written anti-language as in a spoken one, though the changes are in symbols for letters in stead of changing words. Underneath, it is still using the larger language of the speech community, but making it unrecognisable; here is the first paragraph of the Gettysburg Address with increasing degrees of Leetspeak applied:
Fourscor3 and s3v3n y3ars ago our fath3rs brought forth on this contin3nt a n3w nation, conc3iv3d in lib3rty and d3dicat3d to th3 proposition that all m3n ar3 cr3at3d 3qual.
ƒ0µr$(0r3 4n�? $3v3n ¥34r$ 490 0µr ƒ47h3r$ br0µ9h7 ƒ0r7h 0n 7h1$ (0n71n3n7 4 n3w n4710n, (0n(31v3�? 1n £1b3r7¥ 4n�? �?3�?1(473�? 70 7h3 pr0p0$1710n 7h47 4££ m3n 4r3 (r3473�? 3qµ4£.
ƒ0µ®$(0®3 4|\|�? $3\/3|\| ¥34®$ 490 0µ® ƒ47|-|3®$ ß®0µ9|-|7 ƒ0®7|-| 0|\| 7|-|1$ (0|\|71|\|3|\|7 4 |\|3\/\/ |\|4710|\|, (0|\|(31\/3�? 1|\| £1ß3®7¥ 4|\|�? �?3�?1(473�? 70 7|-|3 p®0p0$1710|\| 7|-|47 4££ |\/|3|\| 4®3 (®3473�? 3¶µ4£.
(Translations made using the AlibinoBlackSheep.com Leet Translator)
At 10%, it merely resembles the kind of text commonly used in chat, like capitalising every other letter, or all consonants, and other scripts that many find tedious and annoying. At 50%, the original text can be seen poking through and it is just about possible to decifer it. However, at 100% percent, it has become utterly unrecognisable to me- the only word I can make out without concentration is proposition- p®0p0$1710|\|. It is interesting that a written anti-language seems to display the same characteristics seen in spoken ones, a "conscious counter-reality not a subcultural variant."