In mathematics, parity is the property of an integer of whether it is even or odd. An integer is even if it is divisible by 2, and odd if it is not. For example, −4, 0, and 82 are even numbers, while −3, 5, 23, and 67 are odd numbers.

The above definition of parity applies only to integer numbers, hence it cannot be applied to numbers with decimals or fractions like 1/2 or 4.6978. See the section "Higher mathematics" below for some extensions of the notion of parity to a larger class of "numbers" or in other more general settings.

Even and odd numbers have opposite parities, e.g., 22 (even number) and 13 (odd number) have opposite parities. In particular, the parity of zero is even. Any two consecutive integers have opposite parity. A number (i.e., integer) expressed in the decimal numeral system is even or odd according to whether its last digit is even or odd. That is, if the last digit is 1, 3, 5, 7, or 9, then it is odd; otherwise it is even—as the last digit of any even number is 0, 2, 4, 6, or 8. The same idea will work using any even base. In particular, a number expressed in the binary numeral system is odd if its last digit is 1; and it is even if its last digit is 0. In an odd base, the number is even according to the sum of its digits—it is even if and only if the sum of its digits is even.

An even number is an integer of the form $${\displaystyle x=2k}$$ where k is an integer; an odd number is an integer of the form $${\displaystyle x=2k+1.}$$

An equivalent definition is that an even number is divisible by 2: $${\displaystyle 2\ |\ x}$$ and an odd number is not: $${\displaystyle 2\not |\ x}$$

The sets of even and odd numbers can be defined as following: $${\displaystyle \{2k:k\in \mathbb {Z} \}}$$ $${\displaystyle \{2k+1:k\in \mathbb {Z} \}}$$

The set of even numbers is a prime ideal of ${\displaystyle \mathbb {Z} }$ and the quotient ring ${\displaystyle \mathbb {Z} /2\mathbb {Z} }$ is the field with two elements. Parity can then be defined as the unique ring homomorphism from ${\displaystyle \mathbb {Z} }$ to ${\displaystyle \mathbb {Z} /2\mathbb {Z} }$ where odd numbers are 1 and even numbers are 0. The consequences of this homomorphism are covered below.

The following laws can be verified using the properties of divisibility. They are a special case of rules in modular arithmetic, and are commonly used to check if an equality is likely to be correct by testing the parity of each side. As with ordinary arithmetic, multiplication and addition are commutative and associative in modulo 2 arithmetic, and multiplication is distributive over addition. However, subtraction in modulo 2 is identical to addition, so subtraction also possesses these properties, which is not true for normal integer arithmetic.