An Act of Parliament normally starts as a proposed law known as a Bill. The Bill will then be introduced into the Parliament (House of Commons, House of Representatives, House of Lords or the Senate) for debate and possible approval and enactment.
In some countries, such as Spain and Portugal, a Bill has different names depending on whether it comes from the government (e.g. "Project") or from the Parliament (e.g. "Proposition" or Private Member's Bill).
An Act may be introduced to parliament also by means of a private bill.
Act of Parliament Facts for Kids. Kiddle Encyclopedia.