Agitator is a term for a person that actively supports some ideology or movement with speeches and especially actions.
In the English-speaking world, the term first referred to men who were elected to present complaints of army soldiers, including the New Model Army of Oliver Cromwell, during the English Civil War. They were also known as adjutators.
The practice begun in 1647 when the Long Parliament wanted to either disband the armies or send them to Ireland. Most soldiers did not agree with this and eight of the cavalry regiments elected representatives they called commissioners, who presented the soldiers' complaints, including the fact that their payments were eight weeks late. On April 3 1647 their letter was read in the House of Commons. Before the generals could return to London, parliament again decided to disband the army and settled June 1 as the date that would happen.
Following the example of the Ironsides, other units followed suit and foot soldiers elected Agitators. The Agitators, with two officers from each regiment and the Generals formed a new body called the Army Council which after a rendezvous (meeting) near Newmarket on Friday 4 June 1647 issued "A Solemne Engagement of the Army, under the Command of his Excellency Sir Thomas Fairfax" to Parliament on 8 June making their concerns known, and also the constitution of the Army Council so that Parliament would understand that the discontent was army-wide and had the support of both officers and other ranks. This Engagement was read out to the army at a general rendezvous on 5 June.
A few weeks later there was another rendezvous at while the army was camped at Thriplow Heath near Royston, the soldiers refused the offers made by Parliament, and the agitators demanded a march towards London and the "purging" of the House of Commons.
Later Levellers adopted many of the Agitator's ideas.