Definition - What does Hard Fork mean?
A hard fork in the world of bitcoin and cryptocurrency is a phenomenon in which a change forces certain divergences in the blockchain, as a result of miner or user activity or a change in rules. In the world of cryptocurrency, there are hard forks and soft forks; unlike a soft fork, a hard fork does not resolve automatically according to user trends.
Techopedia explains Hard Fork
Defining and describing hard forks is tricky, partially due to various emerging opinions on what constitutes a hard fork, and partly due to debate on whether a particular change was a hard fork or not. There is widespread consensus that the emergence of Bitcoin Cash as an alternative to Bitcoin Classic was a hard fork – one good definition of a hard fork posted on forums is that in a hard fork, “node consensus diverges permanently” – for example, as a result of Bitcoin Cash, there are now two distinct bitcoin models that are completely separate, leading to the characterization of the change as a hard fork. By contrast, items like the implementation of Segregated Witness are generally called soft forks as they are more backwards-compatible.
Source: https://www.techopedia.com/definition/32912/hard-fork