Sentence:  His life-long friend , Serge Koussevitzky , gave unreservedly of his praise and brilliant performances in Boston , New York , and Washington , <WSD>D.</WSD> C. , to which he added broadcastings and recordings for the whole nation . 
Meaning of the word marked with <WSD> tags:  the district occupied entirely by the city of Washington; chosen by George Washington as the site of the capital of the United States and created out of land ceded by Maryland and Virginia