Block reward reduction is an event that occurs on a blockchain when a certain block height is reached, causing the block reward per block to decrease.
The most common form of block reward reduction is halving, which reduces the block reward by 50%. In Bitcoin, for instance, halving occurs every 210,000 blocks; While in Litecoin, it occurs every 840,000 blocks. Other methods of block reduction include Dash, which reduces its block rewards by 7.14% every 210,240 blocks, and Ethereum Classic, which reduces the block reward by 20% every 5,000,000 blocks.
Check the Halvings Page for scheduled block reduction events.