a law that has been officially accepted by a parliament, especially the British Parliament. Before a law becomes an ‘Act’, when it is still being discussed, it is called a ‘bill’.
From Longman Business Dictionary
Act of ParliamentˌAct of ˈParliament noun (plural Acts of Parliament) [countableC]LAW
a law made by the central national elected body of Britain or of some other countries
New regulations will require an Act of Parliament.