What is an Assembly? 
Quoted from Black's Law 6th Edition Dictionary. ( Black's Law Dictionary is the most widely used law dictionary in the United States Assembly. )
The concourse or meeting together of a considerable number of persons at the same place.  Also the persons so gathered.

Political assemblies are those required by the constitution and laws:  for example, the general assembly.  The lower or more numerous branch of the legislature in many of the states (e.g. N.Y.) is also called the "Assembly" or "House of Assembly."  See also House of Representatives.

Popular assemblies are those where the people meet to deliberate upon their rights;  these are guaranteed by the Constitution.  See Assembly, right of.

Assemblyman.  Member of state Assembly (q.v.)

Assembly, right of.  Right guaranteed by First Amendment, U.S. Constitution, allowing people to meet for any purpose connected with government;  it encompasses meeting to protest governmental policies and actions and the promotion of ideas. See Unlawful Assembly

Assembly, unlawful. The congregating of people which results in antisocial behavior of the group, e.g. blocking a sidewalk, obstructing traffic, littering streets;  but, a law which makes such congregating a crime because people may be annoyed is violative of the right of free assembly.  Coates v. City of Cincinnati, 402 U.S. 611, 91 S.Ct. 1686, 29 L.Ed.2d 214.  See Unlawful assembly.