The Collaborative International Dictionary of English v.0.48:

parity bit \par"i*ty bit`\, n. (Computers)
   The bit within a data structure which is assigned a value of
   1 or 0 so as to make the parity[7] of the data structure odd
   or even. Data structures may or may not have parity bits,
   dpending on whether the system does or does not perform
   parity checking. The most commonly used parity bit is the
   eigth (higher-order) bit of a byte, which is used when data
   transmission uses only the 7 lower-order bits of each byte as
   significant data; some memory systems use a ninth bit as a
   parity bit for each eight bits (one byte) of significant data
   in memory.
   [PJC]

WordNet (r) 3.0 (2006):

parity bit
    n 1: (computer science) a bit that is used in an error detection
         procedure in which a 0 or 1 is added to each group of bits
         so that it will have either an odd number of 1's or an even
         number of 1's; e.g., if the parity is odd then any group of
         bits that arrives with an even number of 1's must contain
         an error [syn: parity bit, parity, check bit]

The Free On-line Dictionary of Computing (30 December 2018):

parity bit

    An extra bit added to a byte or
   word to reveal errors.

   See parity.

   (1996-03-01)