Halving is when the mining reward for a cryptocurrency is split in half. If miners are awarded 100 coins for validating a block, halving would cut the mining reward for 50 coins. Halving does not happen just once, it occurs after a certain number of blocks have been mined. In the case of Bitcoin, halving occurs after every 200,000 blocks have been mined. Halving intervals are set forth in the cryptocurrencies source code, and they occur to control the supply.