Bitcoin halving is a scheduled event, coded into the protocol itself, that cuts the reward miners receive for validating a new block exactly in half. It happens automatically every 210,000 blocks, or roughly every four years.
Why it exists
Bitcoin has a hard supply cap of 21 million coins. Halving is the mechanism that enforces that cap gradually, slowing the rate of new issuance over time rather than releasing all coins at once. Early miners were rewarded 50 BTC per block; that reward has stepped down at each halving since.
Why the market pays attention
A halving reduces the flow of new supply hitting the market from miners, without any change to demand. Historically this supply shock has preceded periods of significant price movement, though correlation with any single halving is not guaranteed and macro conditions vary each cycle.
What it does not do
Halving does not affect coins already in circulation, and it does not change transaction fees or network security directly. Its effect is specifically on the rate of new BTC issuance.