# Definition:Congruence (Number Theory)/Modulo Operation

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## Definition

Let $z \in \R$.

Let $\bmod$ be defined as the modulo operation:

$x \bmod y := \begin{cases} x - y \floor {\dfrac x y} & : y \ne 0 \\ x & : y = 0 \end{cases}$

Then congruence modulo $z$ is the relation on $\R$ defined as:

$\forall x, y \in \R: x \equiv y \pmod z \iff x \bmod z = y \bmod z$

The real number $z$ is called the modulus.

## Notation

The relation $x$ is congruent to $y$ modulo $z$, usually denoted:

$x \equiv y \pmod z$

is also frequently seen denoted as:

$x \equiv y \ \paren {\mathop {\operatorname{modulo} } z}$

Some (usually older) sources render it as:

$x \equiv y \ \paren {\mathop {\operatorname{mod.} } z}$

## Linguistic Note

The word modulo comes from the Latin for with modulus, that is, with measure.