polonium, Po, atomic number 84 -- (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)
petty officer, PO, P.O. -- (a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army)
Po, Po River -- (a European river; flows into the Adriatic Sea)
United States Post Office, US Post Office, Post Office, PO -- (an independent agency of the federal government responsible for mail delivery (and sometimes telecommunications) between individuals and businesses in the United States)