Colonialism is the system in which one country takes control of another land, sends its own people there, rules the local people, and uses the land’s money, crops, or power. The home country does the colonizing; the place it controls is the colony. This is a school-and-news word, a little formal. You meet it in history, in politics, and in arguments about the past and what still follows from it. In today’s English it almost always sounds critical: people who use it are usually saying the system was unfair, violent, or still leaves damage. Older books may talk about “the colonial period” in a colder, more matter-of-fact way, without the -ism. People also stretch it. Writers speak of cultural colonialism or a new colonialism when they mean control that is not a classic overseas empire. That stretch is common in essays; it is looser than the history-book sense. A colony is a place. Colonialism is the system. A colonist is a person who goes to live there. Those are related, not the same word.