The easiest way would be to view imperialism as an ideology, or policy of a country, and expansionism as more vague, but with the simple goal of increasing one's territory, however that may be.

Imperialism is viewed by most historians in this field as both a verb, the actions or forces of imperialism, and a noun, the Imperial state or project. In this understanding you have more nuance and can then see that many subjects of imperialism actually end up assisting in the Imperial Project or are affected by it in unseen ways in the verb sense. While in the noun sense an Imperial state or project has the metropole or the center, that is the imperialist, and the periphery, the colonies, or outer sphere of influence the metropole exercises hegemony over.

An example. India was an Imperial project of GB. As a project, GB is the metropole and India its periphery, the purpose being the exploitation of cash crops, and with the advent of globalism a cheap labor force for goods to send back to the center. Imperialism in action was done via trade companies asserting influence that were then nationalized by the British government. The agents of imperialism being in many cases sepoys of India itself to maintain control. The very population being exploited was part of that exploitation process. This was due to imperialism in action having many unforseen affects on Indian culture, mainly the regimented caste system the British used to organize Indian society benefiting the Indians on top who then had a vested interest in propelling the Imperial project forward.

In this case the British hardly expanded their natural territorial boundaries, but did so in a grand Imperial sense by having a massive area firmly under their exclusive sphere of influence.