imperial adj.

mid-14c., "territory subject to an emperor's rule;" in general "realm, dominion;" late 14c. as "authority of an emperor, supreme power in governing; imperial power," in Middle English generally of the Roman Empire.

From Old French empire "rule, authority, kingdom, imperial rule" (11c.), from Latin imperium "a rule, a command; authority, control, power; supreme power, sole dominion; military authority; a dominion, realm," from imperare "to command," from assimilated form of in- "in" (from PIE root *en "in") + parare "to order, prepare" (from PIE root *pere- (1) "to produce, procure").

Not etymologically restricted to "territory ruled by an emperor," but used that way. The Empire, meaning "the British Empire," first recorded 1772 (it officially devolved into "The Commonwealth" in 1931); before that it meant the Holy Roman Empire (1670s).

[P]roperly an empire is an aggregate of conquered, colonized, or confederated states, each with its own government subordinate or tributary to that of the empire as a whole. [Century Dictionary] 

Empire as the name of a style (especially in reference to a style of dresses with high waistlines) is by 1860, in reference to the affected classicism prevailing in France during the reign of Napoleon I (1804-15). Second Empire is in reference to the rule of Napoleon III of France (1852-70). New York has been called the Empire State since 1834.

1826, "advocacy of empire, devotion to imperial interests," originally in a Napoleonic context, also of Rome and of British foreign policy; from imperial + -ism. At times in British usage (and briefly in U.S.) with a neutral or positive sense relating to national interests or the spread of the benefits of Western civilization, but from the beginning usually more or less a term of reproach. General sense of "one country's rule over another," first recorded 1878. 

It is the old story of 1798, when French republicanism sick of its own folly and misdeeds, became metamorphosed into imperialism, and consoled itself for its incapacity to found domestic freedom by putting an iron yoke upon Europe, and covering it with blood and battle-fields. [Francis Lloyd, St. James's Magazine, January 1842]

The word sharpened its pejorative after 1900, with U.S. revulsion to Philippine insurrection and the publication of J.A. Hobson's book on imperialism, through which the term was taken up in the 1910s by communist writers.

Imperialism is a depraved choice of national life, imposed by self-seeking interests which appeal to the lusts of quantitative acquisitiveness and of forceful domination surviving in a nation from early centuries of animal struggle for existence. Its adoption as a policy implies a deliberate renunciation of that cultivation of the higher inner qualities which for a nation as for an individual constitutes the ascendency of reason over brute impulse. It is the besetting sin of all successful States, and its penalty is unalterable in the order of nature. [J.A. Hobson, "Imperialism: A Study," London, 1902]