Soft-Fork
A soft-fork is a change to the Bitcoin protocol that makes validation rules stricter, meaning that all blocks and transactions that are valid with the new set of rules are also valid with the old ones. Thus any node that does not upgrade to the new protocol would still validate any block or transaction created under the new rules. For example, the addition of Pay to script hash (P2SH) transaction type to the scripting system was made through a soft-fork.