In cryptocurrency mining, difficulty is a measure of how hard it is to find a valid hash for a new block on the blockchain. The difficulty adjusts automatically at regular intervals — every 2,016 blocks on Bitcoin, roughly every two weeks — to ensure that new blocks are produced at a consistent rate regardless of changes in total network hash power. When more miners join the network and hash power increases, the difficulty rises; when miners leave, it decreases.
Example
“Bitcoin's mining difficulty reached a new all-time high after a surge of new ASIC miners came online, making it harder for individual miners to find blocks.”