A private member's bill is a bill introduced into a legislature by a legislator who is not acting on behalf of the executive branch. The designation "private member's bill" is used in most Westminster system jurisdictions, in which a "private member" is any member of parliament (MP) who is not a member of the cabinet (executive). Other labels may be used for the concept in other parliamentary systems; for example, the label member's bill is used in the Scottish Parliament and the New Zealand Parliament, the term private senator's bill is used in the Australian Senate, and the term public bill is used in the Senate of Canada. In legislatures where the executive does not have the right of initiative, such as the United States Congress, the concept does not arise since bills are always introduced by legislators.

A bill is a proposal for a new law, or a proposal to significantly change an existing law. A bill does not become law until it is passed by the legislature and has been, in most cases, approved by the executive. Once a bill has been enacted into law by the legislature, it is called an act of the legislature, or a statute. Bills are introduced in the legislature and are discussed, debated and voted upon.