Leet or eleet (Leet: l337, 3l337 or l33t, 3l33t ), also known as "leetspeak", is an alphabet used primarily on the Internet for the English language. It uses various combinations of ASCII characters to replace Latinate letters. The term is derived from the word "elite", and the usage it describes is a specialized form of symbolic writing. Different "dialects" or varieties of leet are found on different online forums.
Initially, the word l33t was used as an adjective, to primarily describe the behavior or accomplishments of others in the community, with “lame” being its antonym. In that usage leet generally carries the same meaning when referring to either the game prowess, n00b ownage, or, in original usage, hacking expertise of another person. From adjective form its use then expanded to include use as an expletive or interjection in reaction to a demonstration of the former qualities. With the mass proliferation of Internet use in the 1990s into the 21st century, Leet has since become a part of Internet culture and slang. Leet may also be considered a substitution cipher, albeit with much variation from user to user.
Thanks to Wikipedia for me not having to regurgetate the meaning... :-)