Bitcoin halving is the event when after every 4 Years or 210k blocks mined. The reward for bitcoin miners for complete processing the transactions is cut to the half. That means new bitcoins released into to circulations are will be half in quantity simply.
According to the rules, this system will keep running till 2140, like in 2009 the total reward for each chain mined was 50 Bitcoins, and simply after first halving event , it becomes 25 and so on.
Here is the list of complete halving events occurred till this date:
|Event||Date||Block number||Block reward||Total new bitcoins between events|
|Bitcoin launches||3 January 2009||0 (genesis block)||50 new BTC||10,500,000 BTC|
|First halving||28 November 2012||210,000||25 new BTC||5,250,000 BTC|
|Second halving||9 July 2016||420,000||12.5 new BTC||2,625,000 BTC|
|Third halving||Expected week commencing 18 May 2020||630,000||6.25 new BTC||1,312,500 BTC|
|Fourth halving||Expected 2024||740,000||3.125 new BTC||656,250 BTC|
|Fifth halving||Expected 2028||850,000||1.5625 new BTC||328,125 BTC|