Used of a living thing whose body is made of many cells, not just one. You are multicellular; so is a tree, a fish, a mushroom. A bacterium is not — it is one cell. This is a school-science word. Though actually many cells does not mean many kinds of cells; a living thing can be multicellular and still be quite simple. Some living things live in groups that look a bit like one body but are still separate cells, so the edge of the word is not perfectly tidy.