SD is the written short form of South Dakota, one of the fifty US states, in the north of the country, west of Minnesota and east of Wyoming. You will see the two letters on envelopes, forms, maps, and car plates: an address might end Rapid City, SD. In careful speech you still say the full name, South Dakota; people read the letters aloud mainly when they are looking at a list or filling a box that only has room for two letters. Though actually those same two letters do many other jobs in English — a memory card is an SD card, and statistics uses SD for something else — so the meaning hangs on where you see it. In a US address, a state list, or a map of the States, SD is this state.