The short written form of southwest, or of southwestern. You use it where space is tight and the reader already expects a direction: on a map, on a compass, in a weather report ('wind SW 15 km/h'), and in addresses in some cities. Washington, D.C. is the famous case — an address there ends with a quarter of the city, so you write '400 K Street SW'. Some places also use it in the name of a region, like 'SW Florida'. It is written, not spoken: when you read it out, you say the whole word, 'southwest'. It is usually written with two capital letters and no full stops, though older or more formal books sometimes write S.W. In Britain you will also see SW at the start of London postcodes (SW1, SW19), because London's post areas are named by direction from the centre.