In post-colonial and marxist theory one often comes across the word Imperial. What is the meaning of this word? How does it differ from say a monarchy? Are there in fact several kinds of Imperialism? Does classical political theory in the tradition of Locke use this word?
I can only give a brief overview for the Marxist definition of Imperialism as I don't know enough about the term as used in classical political theory. The Wikipedia entry may be better suited for that.
The classical Marxist analysis of Imperialism (though I don't know if this was Lenin, or Luxembourg, or Marx himself) states that the development of capitalism within one nation state sooner or later reaches a limit. Capitalist systems want to grow and will always need more resources, workers or people buying the stuff.
At this point the state tries to secure overseas resources, markets or workforce for its capitalists. This can happen by subjugating and colonizing another part of the world or by forcing other countries to lower tariffs. This is usually called Imperialism.
Note: Imperialism is often simply used as a derogatory term for policies one does not like, or a state or certain policies are called 'Imperialist' in the same way, without bothering with the question of whether the loose definition given above fits. Sometimes Imperialism is simply used as a synonym for the USA and its allies.
Imperial literally means, the desire to create or maintain an empire. An empire is control or dominion over more than one's own land for the purpose of enriching oneself.. or one's own land. In this sense, any nation that uses force to obtain resources from a foreign land is engaging in imperialism. The most blatent modern example is the US. But The UK is possibly the words most notorious imperialist nation. The connotation when used as a pejorative is usually that imperialist nations are living beyond the means that could be supported were they not subjugating others.