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Glyph variant of numeral 0 (zero) with slash

The dotted or slashed zero 0̷ is a representation of the Arabic digit "0" (zero) with a slash or a dot through it. This variant zero glyph is often used to distinguish the digit "zero" ("0") from the Latin script letter "O" anywhere that the distinction needs emphasis, particularly in encoding systems, scientific and engineering applications, computer programming (such as software development), and telecommunications. It thus helps to differentiate characters that would otherwise be homoglyphs. It was commonly used during the punch card era, when programs were typically written out by hand, to avoid ambiguity when the character was later typed on a card punch.

The slashed zero is used in a number of fields in order to avoid confusion with the letter "O". It is used by computer programmers, in recording amateur radio call signs and in military radio, as logs of such contacts tend to contain both letters and numerals. The slashed zero was used on teleprinter circuits for weather applications. In this usage it was sometimes called communications zero. The slashed zero can be used in stoichiometry to avoid confusion with the symbol for oxygen (capital O). The slashed zero is also used in charting and documenting in the medical and healthcare fields to avoid confusion with the letter "O". It also denotes an absence of something (similar to the usage of an "empty set" character), such as a sign or a symptom. Slashed zeroes can also be used on cheques in order to prevent fraud, for example: Changing a 0 to an 8. Slashed zeros are used on New Zealand number plates.

The slashed zero predates computers, and is known to have been used in the twelfth and thirteenth centuries. In the days of the typewriter, there was no key for the slashed zero. Typists could generate it by first typing either an uppercase "O" or a zero and then backspace, followed by typing the slash key. The result would look very much like a slashed zero. It is used in many Baudot teleprinter applications, specifically the keytop and typepallet that combines "P" and slashed zero. on this browser it produces .

In most typographic designs, the slash of a slashed zero usually does not extend past the ellipse. Compare this to the Scandinavian vowel "Ø", the "empty set" symbol "∅" and the diameter symbol ⌀. A convention common on early line printers left zero unornamented but added a tail or hook to the letter O so that it resembled an inverted Q (like U+213A ℺) or cursive capital letter O Andalé Mono has a dotted zero. IBM Plex Mono uses a dotted zero. Source Code Pro and its associated typefaces use a dotted zero. Cascadia Code, the default font for Windows Terminal, Visual Studio, and Visual Studio Code, uses a dotted zero.

The zero with a dot in the center seems to have originated as an option on IBM 3270 display controllers. The dotted zero may appear similar to the Greek letter theta (particularly capital theta, Θ), but the two have different glyphs. In raster fonts, the theta usually has a horizontal line connecting, or nearly touching, the sides of an O; while the dotted zero simply has a dot in the middle. However, on a low definition display, such a form can be confused with a numeral 8. In some fonts the IPA letter for a bilabial click (ʘ) looks similar to the dotted zero. Alternatively, the dot can become a vertical trace, for example by adding a "combining short vertical line overlay" (U+20D3). It may be coded as 0⃓ giving 0⃓. Dotted zero is used on Slovak license plates since 2023.

IBM (and a few other early mainframe makers) used a convention in which the letter O had a slash and the digit 0 did not.