Noun po has 4 senses

• po(n = noun.substance) atomic number 84, polonium - 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; Array
is a kind of metal, metallic element

• po(n = noun.person) p.o., petty officer - a noncommissioned officer in the Navy or Coast Guard with a rank comparable to sergeant in the Army; Array
has particulars: master-at-arms, scpo, senior chief petty officer
is a kind of enlisted officer, noncom, noncommissioned officer

• po(n = noun.object) po river - a European river; flows into the Adriatic Sea; Array
is a part of italia, italian republic, italy
is a kind of river

• po(n = noun.group) post office, united states post office, us post office - an independent agency of the federal government responsible for mail delivery (and sometimes telecommunications) between individuals and businesses in the United States; Array
is a kind of independent agency