A phenomenon that is mostly heard and was even widely used during the end of 2017 is hardfork. A hardfork is a situation where there is migration from an old protocol of a blockchain to a newer one, a hardfork is a permanent migration. When there is a hardfork on any blockchain, previously valid transactions or blocks will no more be valid once nodes running the outdated version refuse to run the newest software protocol. This causes a split in the blockchain whereby different nodes are running different versions of the software protocol.
Hardfork occurs to add new functionality, to correct risks associated with previous blockchain software protocol versions and to reverse transactions, most cases when an hack happens on a blockchain so as to return funds to owners whose tokens were stolen originally.