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.

Leet originated within bulletin board systems (BBS) in the 1980s, where having "elite" status on a BBS allowed a user access to file folders, games, and special chat rooms. The Cult of the Dead Cow hacker collective has been credited with the original coining of the term, in their text-files of that era. One theory is that it was developed to defeat text filters created by BBS or Internet Relay Chat system operators for message boards to discourage the discussion of forbidden topics, like cracking and hacking.

Once reserved for hackers, crackers, and script kiddies, leet later entered the mainstream. Some consider emoticons and ASCII art, like smiley faces, to be leet, while others maintain that leet consists of only symbolic word obfuscation. More obscure forms of leet, involving the use of symbol combinations and almost no letters or numbers, continue to be used for its original purpose of obfuscated communication. It is also sometimes used as a scripting language. Variants of leet have been used to evade censorship for many years; for instance "@$$" (ass) and "$#!+" (shit) are frequently seen to make a word appear censored to the untrained eye but obvious to a person familiar with leet. This enables coders and programmers especially to circumvent filters and speak about topics that would usually get banned. "Hacker" would end up as "H4x0r", for example.

Leet symbols, especially the number 1337, are Internet memes that have spilled over into some culture. Signs that show the numbers "1337" are popular motifs for pictures and are shared widely across the Internet.

Algospeak shares conceptual similarities with leet, albeit with its primary purpose to circumvent algorithmic censorship online, "algospeak" deriving from algo of algorithm and speak. These are euphemisms that aim to evade automated online moderation techniques, especially those that are considered unfair or hindering free speech. One prominent example is using the term "unalive" as opposed to the verb "kill" or even "suicide". Other examples include using "restarted" or "regarded" instead of "retarded" and "seggs" in place of "sex". These phrases are easily understandable to humans, providing either the same general meaning, pronunciation, or shape of the original word. It is furthermore often employed as a more contemporary alternative to leet. The approach has gained more popularity in 2023 and 2024 with pro-Palestinian social media users as the Gaza war intensified, in order to circumvent algorithms used by platforms such as Meta and TikTok.

One of the hallmarks of leet is its unique approach to orthography, using substitutions of other letters, or indeed of characters other than letters, to represent letters in a word. For more casual use of leet, the primary strategy is to use quasi-homoglyphs, symbols that closely resemble (to varying degrees) the letters for which they stand.

The choice of symbol is not fixed: anything the reader can make sense of is valid in leet-speak. Sometimes, a gamer would work around a nickname being already taken (and maybe abandoned as well) by replacing a letter with a similar-looking digit.