ND is the written short form of North Dakota, a state in the north of the United States, along the Canadian border. People write it on letters, forms, maps, and lists of states (for example Fargo, ND). You say the two letters, not a word. On a US address it means that state and nothing else; the same two letters can mean other things in other worlds (a university nickname, a medical note), so the surrounding words tell you. Though actually, outsiders often mix it up with SD (South Dakota) or with NE, which on US mail is a different state.