Answer: A change in the software from a cryptocurrency which might cause the creation of two separate cryptocurrencies.
A hard fork is a change in the software of the cryptocurrency. Because of this, the cryptocurrency will be in maintenance.
If everyone supports the change, no two different cryptocurrencies will arise.
However, when not everyone supports the change, two cryptocurrencies can arise. The blockchain from the two new cryptocurrencies is the same until the hard fork point. After this hard fork, the blockchain splits up into the old version and the changed version. This type of change happened among other things with Bitcoin and Ethereum. Bitcoin Cash and Ethereum classic arose this way.