leetspeak

Leet (or "1337"), also known as eleet, leetspeak, or simply hacker speech, is a system of modified spellings used primarily on the Internet. It often uses character replacements in ways that play on the similarity of their glyphs via reflection or other resemblance. Additionally, it modifies certain words on the basis of a system of suffixes and alternative meanings. There are many dialects or linguistic varieties in different online communities.
The term "leet" is derived from the word elite, used as an adjective to describe skill or accomplishment, especially in the fields of online gaming and computer hacking. The leet lexicon includes spellings of the word as 1337 or leet.

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