Activist:
A zealous member of a party or faction with extremist or extreme tendencies who infiltrates everywhere, takes advantage of every opportunity and is not averse to any method, even violent, to ensure the triumph of his ideological or political aims.
Platform activist:
A person who seeks to promote his or her ideas and points of view through the systematic use of radio call-in shows.
Union activist:
A person who actively participates in the life of a union or trade union movement. The union activist is a grassroots activist if he or she continues to work and devotes volunteer time to trade unionism. Unlike union staff, union activists hold elected office at a higher level.