Hard Fork
From Steem Center
A hard fork is a change to a cryptocurrency protocol that makes previously invalid blocks/transactions valid, and therefore requires all users / software to upgrade. It could be any alteration to a coin which changes the block structure (including block hash), difficulty rules, or increases the set of valid transactions. You can think of a hard fork as a change of rules.
Unlike a softfork, a hardfork is not backward-compatible: unupdated nodes will not recognise the new blocks as valid.
