Soft Fork, Soft-Forking Change
This definition comes from the technical glossary.
Definition
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.
Synonyms
- Soft fork
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) 
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    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 
Links
- 
    Soft fork — Developer Guide 
- 
    Soft fork — Bitcoin Wiki 
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    What is a soft fork? — Bitcoin StackExchange