A halving is a programmed event in which the block reward given to miners is reduced by 50%. In Bitcoin, this occurs approximately every 210,000 blocks, or roughly every four years.
Bitcoin halving history: 2012 halving reduced reward from 50 to 25 BTC, 2016 halving reduced from 25 to 12.5 BTC, 2020 halving reduced from 12.5 to 6.25 BTC, and 2024 halving reduced from 6.25 to 3.125 BTC.
Economic impact: Halvings reduce new supply entering the market, historically followed by price appreciation, create supply shock if demand remains constant, and mining becomes less profitable requiring efficiency improvements.
Other cryptocurrencies with halving mechanisms include Litecoin (every 840,000 blocks), Bitcoin Cash, and Zcash.