What is Petty Officer?

Legal Definition
A petty officer (PO) is a non-commissioned officer in many navies and is given the NATO rank denotion OR-6. They are equal in rank to sergeant in the British Army and Royal Air Force. A petty officer is superior in rank to leading rate and subordinate to chief petty officer, in the case of the British armed forces.
-- Wikipedia
Legal Definition
Inferior officers in the naval service, of various ranks and kinds, corresponding to the non-commissioned officers in the army. See U. S. v. Fuller, 160 U. S. 593,16 Sup. Ct. 386, 40 L. Ed. 549. As to petty "Average," "Constable," and "Sessions," see those titles.
-- Black's Law Dictionary