Leet speak, also known as hackspeak or simply leet, is the substitution of a word's letters with numbers or special characters. "Leet" is derived from the word "elite," which refers to the hackers who originally turned leet speak into a sort of cult language in the 1980s. Over time, the label "leet" has broadened to include any term used by programmers that is a mystery to everyone else.
In traditional leet speak, characters and combinations of characters are often chosen to resemble the letters they replace so the resulting word is visually similar. Many people remember learning that when 0.7734 is entered into a calculator and looked at upside down, the numbers appear to spell the word hello. Leet speak is a somewhat similar concept; for example the word hacker in leet speak might look like this: |-|@k3r.
Although leet is human-readable (albeit with some difficulty), it is difficult for a computer to decode. For this reason, a spammer might use leet speak to bypass text parsers and encrypt unsolicted email. Spam filtering programs have become good at detecting leet speak in subject lines and rejecting e-mail messages that contain it, but they often have less success detecting it in e-mail text.
Leet speak is sometimes used to conceal the content of an online conversation from authorities, but it is also used sometimes just for the fun of it. For example, "pwn" is a leet that became a popular internet meme. It's thought that the leet probably originated from a typo when, in the excitement of a decisive win over an opponent, a gamer mistyped "own" and instead sent the message "Now I pwn you!"
Today, many people use leet to create strong passwords that are easy for the originator to remember, but difficult for intruders to steal with a dictionary attack. Text that combines letters and numerals is sometimes called alphanumerish.