difficulty
A number that controls how hard it is to mine a new Bitcoin block, adjusted every 2016 blocks to keep the average block time near ten minutes.
Bitcoin's difficulty is a target number embedded in the protocol. When a miner wants to add a new block, the cryptographic hash of that block's header must be numerically smaller than the current target. A lower target means fewer valid hashes exist, so miners must perform more work on average before finding one.
Every 2016 blocks, roughly every two weeks, every node independently recalculates the difficulty. The network compares how long those 2016 blocks actually took against the intended 20160 minutes. If mining found blocks faster, difficulty rises; if slower, it falls. This automatic adjustment keeps the average block time close to ten minutes regardless of how much mining power joins or leaves the network.
The difficulty mechanism is directly linked to halving and block rewards. As mining hardware improves or more miners join, difficulty rises to compensate, preserving the predictable issuance schedule. If miners leave en masse, difficulty drops so that remaining miners can still produce blocks at a sustainable rate.