Imperialism is a type of government that seeks to increase its size, either by forcing (through war) or influencing (through politics) other countries to submit to their rule.
The sun never sets on the Roman Empire, or so went the saying when Rome controlled the majority of Europe 2000 years ago. Empire has the same Latin root as imperialism, and the Roman Empire grew to be so vast because they invaded, conquered, and developed everywhere they could. Until Rome fell, that is. Imperialism isn’t always violent, but it’s usually used in a negative sense, so watch for secret agents if you’re talking about your country’s policy of imperialism.