A softfork is a change to the bitcoin protocol wherein only previously valid blocks/transactions are made invalid. Since old nodes will recognise the new blocks as valid, a softfork is backward-compatible.

Not to be confused with: Fork (a regular fork where all nodes follow the same consensus rules, so the fork is resolved once one chain has more proof of work than another), Hard fork (a permanent divergence in the block chain caused by non-upgraded nodes not following new consensus rules), Software fork (when one or more developers permanently develops a codebase separately from other developers), Git fork (when one or more developers temporarily develops a codebase separately from other developers