WordNet sense 1 (a radioactive metallic element that is similar to tellurium and bismuth; occurs in uranium ores but can be produced by bombarding bismuth with neutrons in a nuclear reactor):

polonium

WordNet sense 2 (a noncommissioned officer in the navy with a rank comparable to sergeant in the army):

P.O., petty officer

WordNet sense 3 (a European river; flows into the Adriatic Sea):

Po River

WordNet sense 4 (an independent agency of the federal government responsible for mail delivery (and sometimes telecommunications) between individuals and businesses in the United States):

post office, United States Post Office, US Post Office