Bitcoin Halving.

A scheduled event roughly every four years where the reward miners earn for adding a new Bitcoin block gets cut in half. It's the mechanism that caps total supply at 21 million BTC and steadily slows the rate of new issuance. The honest caveat: halvings are often hyped as automatic price catalysts; the supply effect is real but past price runs around halvings were driven by many things at once, and the sample size is tiny (only four halvings so far).