Glossary
Difficulty
A parameter that adjusts the work needed to mine a valid block so that blocks arrive at the protocol's target rate regardless of total network hashrate.
PoW chains target a fixed average block time — 10 minutes on Bitcoin, ~13 seconds on Ethereum before the Merge. Difficulty is the knob that keeps that target constant: if blocks come in too fast, difficulty rises; too slow, it falls.
A valid block must hash to a value below 2^256 / difficulty.
Doubling difficulty halves the chance per hash attempt, so a doubling
of network hashrate is exactly cancelled by the next adjustment.