A macrocosm is a very large, complicated system that you look at as one whole thing. The first meaning is the universe itself — everything that exists, taken together. People also use it for a whole society, a whole culture, or any huge human world, when they want you to see the big picture rather than the little pieces. It is a bookish word. In ordinary talk most people would just say “the universe” or “the whole system.” You will often meet it next to its partner microcosm: a small thing that seems to copy the large one. If a classroom is a microcosm of society, then society is the macrocosm. Teachers and writers like this pair; it is not a word you need at the market.