Imperialism can also refer more generally to a country's efforts to have a lot of power and influence over other countries, esp. in political and economic matters.
Source: Cambridge International Dictionary of English
imperial government, authority, or system
the policy, practice, or advocacy of extending the power and dominion of a nation especially by direct territorial acquisitions or by gaining indirect control over the political or economic life of other areas; broadly : the extension or imposition of power, authority, or influence (union imperialism)
Source: Merriam-Webster's WWWebster Dictionary
Machiavelli and imperialism
Machiavelli does not suggest anywhere in The Prince that a prince should always be expanding his empire. The Prince is only concerned with the proper leading of one principality only.