A hard fork is a process of splitting any currency's Blockchain. It may be done for several reasons, such as fixing serious security loopholes in a cryptocurrency's source code, implementing new features and reverting transactions (as was the case for Ethereum's hard fork after The DAO was hacked). Hard forking requires a consensus of the majority of the nodes within the network of a cryptocurrency, after which they all switch to the new Blockchain, leaving the older version behind.