A type of fork caused by a backwards compatible update to the node software. Typically a tightening of the ruleset so old nodes can continue to process blocks, however blocks created by nodes running the new software will be rejected by nodes running the old software. Over time, this leads to old nodes upgrading and their fork eventually become obsolete.
The bitcoin community agrees that only soft forks are acceptable moving forward and there should never be a hard fork. A famous example of a soft fork is Bitcoin’s Segwit upgrade.
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