PO

from WordNet (r) 3.0 (2006)

Po
    n 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 [syn:
         {polonium}, {Po}, {atomic number 84}]
    2: a noncommissioned officer in the Navy or Coast Guard with a
       rank comparable to sergeant in the Army [syn: {petty
       officer}, {PO}, {P.O.}]
    3: a European river; flows into the Adriatic Sea [syn: {Po}, {Po
       River}]
    4: 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}]

from The Collaborative International Dictionary of English v.0.48

Phosphoryl \Phos"phor*yl\ (f[o^]s"f[o^]r*[i^]l), n. [Phosphorus
   + -yl.] (Chem.)
   The radical {PO}, regarded as the typical nucleus of certain
   compounds.
   [1913 Webster]

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

PO
       Portable Object (MO)