Halving
In short: A halving is the event where Bitcoin rewards are cut in half: the number of new bitcoins a miner receives for adding a block drops by 50 percent.
A halving is the event where Bitcoin rewards are cut in half: the number of new bitcoins a miner receives for adding a block drops by 50 percent. It happens automatically every 210,000 blocks, which works out to roughly every four years.
Why it matters
Block rewards are the only way new bitcoins enter circulation, so each halving slows the growth of supply as the total approaches the fixed cap of 21 million coins. The schedule is written into the protocol and known in advance, which makes Bitcoin's issuance predictable. Because the reward keeps shrinking, the protocol relies more and more on transaction fees to pay miners. The effect of halvings on price is widely debated, and past cycles are no guide to future ones.
Example
When Bitcoin launched, each new block paid its miner 50 BTC. After the first halving the reward became 25 BTC, after the second 12.5 BTC, and it keeps halving on the same schedule. Transaction fees are paid on top of this reward and are not cut by the halving.