The Collaborative International Dictionary of English v.0.48:

Unlawful \Un*law"ful\, a.
   Not lawful; contrary to law. -- Un*law"ful*ly, adv. --
   Un*law"ful*ness, n.
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   Unlawful assembly. (Law) See under Assembly.
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The Collaborative International Dictionary of English v.0.48:

Assembly \As*sem"bly\, n.; pl. Assemblies. [F. assembl['e]e,
   fr. assembler. See Assemble.]
   1. A company of persons collected together in one place, and
      usually for some common purpose, esp. for deliberation and
      legislation, for worship, or for social entertainment.
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   2. A collection of inanimate objects. [Obs.] --Howell.
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   3. (Mil.) A beat of the drum or sound of the bugle as a
      signal to troops to assemble.
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   Note: In some of the United States, the legislature, or the
         popular branch of it, is called the Assembly, or the
         General Assembly. In the Presbyterian Church, the
         General Assembly is the highest ecclesiastical
         tribunal, composed of ministers and ruling elders
         delegated from each presbytery; as, the General
         Assembly of the Presbyterian Church in the United
         States, or of Scotland.
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   Assembly room, a room in which persons assemble, especially
      for dancing.

   Unlawful assembly (Law), a meeting of three or more persons
      on a common plan, in such a way as to cause a reasonable
      apprehension that they will disturb the peace
      tumultuously.

   Westminster Assembly, a convocation, consisting chiefly of
      divines, which, by act of Parliament, assembled July 1,
      1643, and remained in session some years. It framed the
      "Confession of Faith," the "Larger Catechism," and the
      "Shorter Catechism," which are still received as authority
      by Presbyterians, and are substantially accepted by
      Congregationalists.
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   Syn: See Assemblage.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNLAWFUL ASSEMBLY, crim. law. A disturbance of the public peace by three or
more persons who meet together with an intent mutually to assist each other
in the execution of some unlawful enterprise of a private nature, with force
and violence; if they move forward towards its execution, it is then a rout
(q.v.) and if they actually execute their design, it amounts to a riot.
(q.v.) 4 Bl. Com. 140; 1 Russ. on Cr. 254; Hawk. c. 65, s. 9; Com. Dig.
Forcible Entry, D 10; Vin. Abr. Riots, &c., A.