Definition
The nonce (number used once) is the primary variable that miners modify during the mining process. By incrementing or changing the nonce, the miner produces a completely different hash output from the block header data.
Since the nonce is only 32 bits (about 4.3 billion possible values), modern ASIC miners can exhaust the entire nonce space in less than a second. Miners therefore also modify the extra nonce field in the coinbase transaction and adjust timestamps to generate additional unique block header inputs.
In Simple Terms
A number miners change to produce different hash results when searching for a valid block.
A 32-bit number in the block header that miners change repeatedly to produce different hash outputs while trying to find a hash below the target value.