The dictionary definiton you gave is useless for political analysis, hence we use more accurate and precise definitions when in the domain.

An example of why your definition is useless is that it cannot be used to distinguish between imperialism and anti-imperialism. If imperilaism extends a country's power, then it inherently extends it into some other country's sphere of influence. When that country fights back, they are inherently extending their influence. Calling both imperialism results in zero understanding that couldn't have been achieved without using the word and instead using the words force, expansion, violence, or many other words.

Imperialism is something specific and fighting against imperialism is not imperialism.