A bit (0 or 1) added to a binary number (such as 1001100) to help check if the number is received correctly.
For even parity all the "1" bits are counted:
• if the count is odd then the parity bit is set to 1 (making an even number of 1s)
• if the count is even then the parity bit is set to 0 (also an even number of 1s)
Examples:
1001100 has an odd number of 1s, so the parity bit is set to 1, giving 10011001
1010011 has an even number of 1s, so the parity bit is set to 0, giving 10100110
So, including the parity bit, all binary numbers will have an even number of 1s.
If we receive a number that breaks this rule then some error happened and the number can be sent again.
There is also odd parity: same idea, but the parity bit is set to always have an odd number of 1s.
Note: this is a good way to check if there is a single error in the number, but won't work if there are two errors!