0 (zero; BrE: or AmE: ) is both a number and the numerical digit used to represent that number in numerals. It fulfills a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, 0 is used as a placeholder in place value systems. Names for the number 0 in English include zero, nought or (US) naught , nil, or — in contexts where at least one adjacent digit distinguishes it from the letter "O" — oh or o . Informal or slang terms for zero include zilch and zip. Ought and aught , as well as cipher, have also been used historically.

Signed zero is zero with an associated sign. In ordinary arithmetic, the number 0 does not have a sign, so that -0, +0 and 0 are identical. However, in  computing, some number representations allow for the existence of two zeros, often denoted by -0 (negative zero) and +0 (positive zero), regarded as equal by the numerical comparison operations but with possible different behaviors in particular operations. This occurs in the sign and magnitude and ones' complement signed number representations for integers, and in most floating-point number representations. The number 0 is usually encoded as +0, but can be represented by either +0 or -0.