a person who stirs up others in order to upset the status quo and further a political, social, or other cause

The boss said he would fire any union agitators.

a machine or device for agitating and mixing.

noun

adjy , advocate , agent , anarchist , champion , demagogue , disrupter , dissident , dogmatist , fighter , firebrand * , fomenter , heretic , incendiary , inciter , instigator , leftist , malcontent , mover , partisan , propagandist , provocateur , pusher , rabble-rouser , radical , reactionary , rebel , reformer , revisionist , revolutionary , ringleader , sparkplug , troublemaker , wave maker , zealot