A parity bit is an extra bit transmitted with each byte of data. The value of the extra bit is chosen to ensure that the number of 1s in the nine bits were odd (odd parity) or even (even parity). If the parity is wrong, the data had been altered, and should be rejected.
This mechanism is frequently used with modem connections.
Parity bits are a relatively expensive form of integrity protection. They increase the size of the message by at least 12.5%. Worse, they may not detect multiple errors in the same byte. While this mechanism can be extended to detect such errors by using additional parity bits, the cost is increased yet again.