Many cryptocurrencies have a limited number of coins that will ever be produced. New coins are issued as a block reward to users who help validate new data being added to the blockchain (mining). Halving is the periodic reduction of the rate of new coins issued. Once the set coin limit is reached, no more coins will be produced.

The number of coins given to Bitcoin miners is set to decrease by 50% approximately every four years. Once 21 million bitcoins have been issued (~ year 2140), no more bitcoins will be created.