Definition
Difficulty is a measure associated with the target a proof-of-work hash must satisfy. A more restrictive target makes successful block discovery less likely per attempt. Difficulty is not the price of the coin, the complexity of individual transactions or a measure of how hard a mathematical puzzle is to understand.
How It Changes
Many networks adjust their target to keep average block production near a desired pace as effective hash power changes. Bitcoin updates difficulty at intervals defined by its protocol; other networks use different schedules and formulas. Block discovery remains probabilistic, so actual intervals vary even when the long-run target is stable.
Key Considerations
For a miner with unchanged hardware, higher difficulty generally reduces expected block discoveries relative to the work performed. Profitability also depends on rewards, fees, asset prices and costs. Difficulty and hash rate are related but distinct: one is a protocol parameter or derived measure, while the other is computational throughput. Figures from different algorithms or networks should not be compared as if they used the same baseline.