trigger law

Short definition from Wiktionary

Noun

1. 1
   A law which is not active until a particular event comes to pass, converting the inactive law into an active law on the books.
2. 2
   A law which is not active until a particular event comes to pass, converting the inactive law into an active law on the books: An unenforceable law which awaits conditions that allow its enforcement to come to pass and then automatically comes active on the books.