A place in the electricity network where power is taken from the high-voltage lines coming from the power station and changed to a lower voltage, so that it can be sent on to homes, shops and factories. Electricity travels long distances at very high voltage, which is efficient but far too dangerous for a house; the substation is the halfway point where that voltage is stepped down and the current is switched and directed. You have almost certainly walked past one: a fenced yard full of grey metal towers and thick insulators that hums, or a small locked brick building on the corner of a street, usually with a warning sign about high voltage. It is a countable noun, and you will meet it in the news whenever there is a blackout, because a fault at a substation can leave a whole district dark. The word is also used, less often, for a small branch office of a larger organisation — a police substation, for instance — but the electrical meaning is the usual one.