• (n.) One who holds to a belief in the progression of society toward perfection. • (n.) One who maintains the doctrine of progression in organic forms; -- opposed to uniformitarian.
Pro·gres'sion·ist noun 1. One who holds to a belief in the progression of society toward perfection. 2. One who maintains the doctrine of progression in organic forms; -- opposed to uniformitarian . H. Spencer. 
a person who believes in progress, as of humankind or society.