Soft Fork

A soft fork occurs when a blockchain protocol changes, but the changes are compatible with the prior protocol version, meaning nodes do not have to upgrade to remain on the network.

A soft fork differs from a hard fork in that the former only introduces protocol changes considered 'backward compatible', i.e., the changes don't conflict with the existing protocol rules, and nodes are not forced to upgrade their software to stay connected to the network. In the case of a hard fork, some or all of the changes are not backwards compatible, meaning that nodes must upgrade or be unable to connect to the new protocol version.