A number placed in coinbase script and incremented by a miner each time the nonce 32-bit integer overflows. This is not the required way to continue mining when nonce overflows. One can also change the merkle tree of transactions or change a public key used for collecting a block reward. See Nonce .
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc