Any bill that has been considered by a congress and gone on to become law is an act of congress. The United States Congress passes legislation which is then approved or vetoed by the executive branch. Other congresses, in other nations, have different specific procedures for constructing, evaluating, and approving legislation. An act of congress is often subject to judicial review, and the specific topics addressed by each piece of legislation are generally limited by a constitution or similar fundamental document.
In a typical legislature, a single elected representative has the authority to present a bill for consideration. In many cases, however, a group of legislators will jointly author and introduce a bill in order to increase its chances of success. In some cases, citizens or the executive branch also have the ability to introduce legislation for consideration.
Once a piece of legislation has been introduced, it will typically be evaluated by a subset of the whole congress. In the United States Congress, each chamber has a series of committees. These committees each have responsibility for one or more areas of legislative activity, and a measure destined to become an act of Congress will usually emerge from a committee, where it has been evaluated and often modified as the members of the committee deem appropriate.
The formal approval of a piece of legislation by the whole of a congress, including passage in each relevant house, is the next step in the passage of an act of congress. In some cases, the executive branch may attempt to veto a piece of legislation, but most congresses have the ability to override a veto if enough legislators agree to do so. Judicial review, such as that carried out by the Supreme Court of the United States, may overturn or modify an act of Congress after passage.
Individual acts of congress vary widely in scope and significance, as a few examples from the United States Congress demonstrate. A single act established the United States Treasury. Major economic programs, such as Social Security, can be put in place by a single act of Congress as long as they are not found to be in violation of the United States Constitution.
In other cases, an act of congress may have a very narrow or purely symbolic function. Some acts of congress have created minor holidays or honored small groups of people without making any significant change to the legal code. In other cases, an act of congress may deal with a single projects in one single state or region.