What is Assembly?

Legal Definition
The union of a number of persons in the same place. There are several kinds of assemblies.

2. Political assemblies, or those authorized by the constitution and laws; for example, the general assembly, which includes the senate and house of representatives; the meeting of the electors of the president and vice-president of the United States, may also be called an assembly.

3. Popular assemblies are those where the people meet to deliberate upon their rights; these are guaranteed by the constitution. Const. U. S. Amend. art. 1 Const. of Penn. art. 9, s. 20.

4. Unlawful assemblies. An unlawful assembly is the meeting of three or more persons to do an unlawful act, although they may not carry their purpose into execution. It differs from a riot or rout, (q. v.) because in each of the latter-cases there is some act done besides the simple meeting.
-- Bouviers Law Dictionary
Legal Definition
The concourse or meeting together of a considerable number of persons at the same place. Also the persons so gathered. Popular assemblies are those where the people meet to deliberate upon their rights ; these are guaranteed by the constitution. Const. U. S. Amend, art 1. The lower or more numerous branch of the legislature in many of the states is also called the "Assembly" or "House of Assembly," but the term seems to be an appropriate one to designate any political meeting required to be held by law.
See also
-- Black's Law Dictionary
Legal Definition
A meeting of several persons.
-- Ballentine's Law Dictionary