In the farm-and-factory sense, a separator is a machine that splits mixed stuff apart — especially cream from milk, or liquid from solids. A dairy separator spins milk so the cream comes off. Factories that make juice, oil, or metal use separators too. It is a technical word: farmers, food plants, and engineers say it. You would not use it for a person, and it is not the everyday word for a wall or a fence. People have stretched it a long way since then. Anything that stands between two things and keeps them apart may be called a separator: a line on a page, a mark between numbers in a date, a character in a computer file. The machine is the older, more physical sense; the “thing in between” sense is now very common.