Definition of "Po" :

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

noun: a European river; flows into the Adriatic Sea

noun: a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army

noun: 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