In mathematics, the **absolute value**, or **modulus** (UK), of a number is that number without a negative sign. So, for example, 3 is the absolute value of both 3 and -3.

It can be defined as follows: For any real number *a*, the **absolute value** of *a* (denoted **| a|**) is equal to

The absolute value can be regarded as the *distance* of a number from zero; indeed the notion of distance in mathematics is a generalisation of the properties of the absolute value. It is thus a concept useful to scientists, for whom it serves as a measure of the **magnitude** of any quantity, whether scalar or vector.

The absolute value has the following properties:

- |
*a*| ≥ 0 - |
*a*| = 0 if and only if*a*= 0. - |
*ab*| = |*a*||*b*| - |
*a/b*| = |*a*| / |*b*| (if*b*≠ 0) - |
*a*+*b*| ≤ |*a*| + |*b*| - |
*a*-*b*| ≥ ||*a*| - |*b*|| - |
*a*| ≤*b*if and only if -*b*≤*a*≤*b*

- |
*x*- 3| ≤ 9 - -9 ≤
*x*-3 ≤ 9 - -6 ≤
*x*≤ 12

For a complex number *z* = *a* + *ib*, one defines the absolute value or *modulus* to be |*z*| = √(*a*^{2} + *b*^{2}) = √ (*z* *z*^{*}) (see square root and complex conjugate). This notion of absolute value shares the properties 1-6 from above. If one interprets *z* as a point in the plane, then |*z*| is the distance of *z* to the origin.

It is useful to think of the expression |*x* - *y*| as the *distance* between the two numbers *x* and *y* (on the real number line if *x* and *y* are real, and in the complex plane if *x* and *y* are complex). By using this notion of distance, both the set of real numbers and the set of complex numbers become metric spaces.

The operation is not reversible because either negative or non-negative number or becomes the same non-negative number.

If the absolute value would not be a standard function **Abs** in Pascal it could be easily computed using the following code:

program absolute_value; var n: integer; beginread (n); if n < 0 then n := -n; writeln (n)end.