What is Rebellious Assembly?

Legal Definition
In English law. A gathering of twelve persons or more, intending, going about or practicing unlawfully and of their own authority to change any laws of the realm; or to destroy the enclosure of any park or ground enclosed, banks of fish-ponds, pools, conduits, etc., to the intent the same shall remain void; or that they shall have way in any of the said grounds; or to destroy the dcer in any park, fish in ponds, coneys in any warren, dove-houses, etc.; or to burn sacks of corn; or to abate rents or prices of victuals, etc. See Cowell.
-- Black's Law Dictionary