Soft Fork
A backward-compatible blockchain protocol upgrade that does not create a permanent chain split.
What is Soft Fork?
A soft fork is a change to a blockchain's protocol rules that is backward-compatible: blocks produced under the new rules are still accepted by nodes running the old rules, because the new rules are a subset or stricter version of the old ones. This means that not all nodes need to upgrade simultaneously — as long as the majority of mining or validating power adopts the new rules, the chain remains unified. Nodes running old software may continue to participate in block validation without splitting the chain, though they cannot take advantage of new features. Soft forks are generally considered less disruptive than hard forks, which create permanent chain splits. A well-known Bitcoin soft fork was Segregated Witness (SegWit) in 2017, which changed how transaction data is structured to increase block capacity and fix transaction malleability without requiring all nodes to upgrade.
Example
Bitcoin's SegWit (BIP 141) soft fork was activated in August 2017. It restructured transaction data so that signature data ('witnesses') were separated from the transaction body, effectively increasing usable block capacity. Older nodes that had not upgraded still accepted SegWit blocks, maintaining network unity and avoiding a chain split.