The "halving" or "halvening" occurs when the rate of creating new tokens of a cryptocurrency is cut in half. It mostly refers to Bitcoin's halving events.

When Satoshi Nakamura created Bitcoin they wanted to build in a mechanism that protected Bitcoin from inflation. They wanted the value of a bitcoin to increase over time, not decrease. Therefore the rate that new bitcoins are created decreases over time until it hits a maximum limit of 21 million coins.

When Bitcoin was created in 2008, 50 bitcoins were rewarded to miners every ten minutes when a new block was added to the blockchain. 210,000 blocks later, in 2012 a halving occured, and the reward decreased to 25 bitcoins. Two more halvings occured in 2016 and 2020 bringing the reward down to 12.5 and 6.25 bitcoins respectively.

There will be 32 total halvings for bitcoin, when the number of bitcoins will reach its maximum of 21 million.