Difficulty adjustment
Also · Mining difficulty · Retarget
Every 2,016 blocks, Bitcoin recomputes its proof-of-work target so that blocks keep arriving every 10 minutes on average, whatever the hashrate.
Bitcoin aims for 1 block every 10 minutes. If more machines join the network, blocks come faster. To compensate, every 2,016 blocks, about 2 weeks, each node compares how long the last period actually took with the 14 days it should have taken, and scales the difficulty accordingly. Faster than planned, difficulty goes up. Slower, it goes down.
A single adjustment is capped at a factor of 4 in either direction, so a sudden shock can take several periods to absorb. The mechanism is what protects the issuance schedule. Without it, better hardware would mint the 21 million coins far ahead of time and break the monetary policy.
In French · Ajustement de la difficulté