Halving is when miners certain amount of reward for creating block chain of cryptocurrencies is split into half. The block reward for miners keep on halving until the reward for each block becomes zero. As part of bitcoin issuance if miners are awarded with 100 bitcoins for creating a bitcoin block, halving will cut down reward to 50 bitcoins. Halving does not happen once, but occurs after certain blocks have been mined. As in case of Bitcoin, halving happen after each 200,000 blocks have been mined. Halving intervals are set in cryptocurrencies code to control the supply.