NW is the written short form of northwest or northwestern: the direction between north and west, or the part of a place that lies that way. You see it on maps, weather reports, and addresses. People write NW; when they talk they usually say “northwest.” In London, NW is also a postcode area for the north-west of the city, so NW5 on an envelope means that part of London, not a random compass point. Though actually, on a compass NW is a direction; on a British letter it may simply be part of the address. Look at the rest of the line before you decide.