Wordnet 3.0
NOUN (1)
1. 
 an independent agency of the federal government responsible for mail delivery (and sometimes telecommunications) between individuals and businesses in the United States; 
[syn: United States Post Office, US Post Office, Post Office, PO]
WordNet (r) 3.0 (2006):
United States Post Office
    n 1: an independent agency of the federal government responsible
         for mail delivery (and sometimes telecommunications)
         between individuals and businesses in the United States
         [syn: United States Post Office, US Post Office, Post
         Office, PO]