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An informal or coded writing system used primarily in Internet-based communications in which certain ASCII characters are substituted for standard letters that have a similar appearance, such as 3 for E, 0 for O, 1 for I or L, etc. We started writing in leet so our parents wouldn't be able to understand what we were saying if they saw it. A: "WH3r3 Sh0uLd w3 m337 UP 70N19h7?" B: "Just write normally, dude. I don't understand leet."