Halving: what it is and how it changes bitcoin's supply

A halving (from “to halve”) is a protocol-level event in which the reward for mining a block is cut in half. For bitcoin it happens roughly every four years, after a fixed number of blocks has been mined. As a result, the issuance of new coins declines and total supply approaches a predetermined cap of 21 million.

A halving affects the supply of new coins and miners' earnings, but its impact on price is disputed: the market knows the date in advance, so it may price it in earlier. The few cases so far do not prove a lasting pattern. Halvings also apply to some other cryptocurrencies, but not all of them, because many projects have a different issuance model.