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New bitcoins are issued by the Bitcoin network every 10 minutes. For the first four years of Bitcoin's existence, the amount of new bitcoins issued every 10 minutes was 50. Every four years, this number is cut in half. The day the amount halves is called a "halving" or "halvening".
The halving decreases the amount of new bitcoins generated per block. This means the supply of new bitcoins is lower, making buying more expensive.