hard fork
A non-backward-compatible change to Bitcoin's rules that creates a permanent split if not everyone upgrades.
soft fork
Definition
A soft fork changes Bitcoin’s rules so that blocks valid under the new rules are also valid under the old ones. Older nodes continue to work, but they don’t enforce the new constraints.
Example
SegWit activated in 2017 as a soft fork, meaning nodes that didn't upgrade could still process transactions.