A nonce is an arbitrary, pseudo-random number used only once in a cryptographic hash function. The word “nonce” stands for number only once.
For Bitcoin, a nonce appears on the tail-end of the block header. It is the number that blockchain miners are solving for, and the number that causes the hash to fall under the target so that the block to be successfully mined.
Finding a successful nonce is difficult, and it is a significant barrier to entry for those new to Bitcoin mining. Finding a nonce at a rate fast enough to compete with other miners requires an incredibly powerful computer, which is why many look to mining pools to compete.