Mining Difficulty
A network parameter that adjusts how hard it is to find a valid block, keeping the average time between blocks roughly constant as mining power changes.
Difficulty is the network's thermostat. Bitcoin retargets it every 2,016 blocks — about a fortnight — so that blocks keep arriving roughly every ten minutes whether the hash rate has doubled or halved. Because it adjusts on a schedule rather than continuously, difficulty is a smoothed record of hash rate rather than an independent quantity.