Imperialism is very similar to colonialism, with one major difference: colonial powers settle the countries of which they gain control, while imperial powers do not. The term “imperialism” does not seem to exist prior to the 1800s. Nineteenth-century imperialism was spurred in large part by the Industrial Revolution. The development of new industrial economies in the 1700s and 1800s necessitated the acquisition of raw materials and the desire to gain control of marketplaces; thus, by the mid-1800s, imperialistic actions of strong nations (most notably European nations) started to become policy.
1 Definition is from America: Pathways to the Present (New Jersey: Pearson Prentice Hall, 2005, p. 981).