A petty officer (PO) is a non-commissioned officer in many navies and is given the NATO rank denotion OR-5. In many nations, they are typically equal to a corporal or sergeant in comparison to other military branches. Often they may be superior to a seaman, generally the (or one of the) lowest ranks in a navy, and subordinate to a more senior non-commissioned officer, such as a chief petty officer.
The modern petty officer dates back to the Age of Sail. Petty officers rank between naval officers (both commissioned and warrant) and most enlisted sailors. These were men with some claim to officer rank, sufficient to distinguish them from ordinary ratings, without raising them so high as the sea officers. Several were warrant officers, in the literal sense of being appointed by warrant, and like the warrant sea officers, their superiors, they were usually among the specialists of the ship's company. The Oxford English Dictionary suggests that the title derives from the Anglo-Norman and Middle French "petit", meaning "of small size, small, little".
Two of the petty officer's rates, midshipman and master's mate, were a superior petty officer with a more general authority, but they remained no more than ratings. However, it was quite possible for a warrant officer (such as the armourer), in his role as a superior officer, to be court-martialed for striking a midshipman. This is because both were regarded as future sea officers, with the all-important social distinction of having the right to walk the quarterdeck. Midshipmen wore distinctive uniforms, master's mates dressed respectably, and both behaved like officers. The master's mate rating evolved into the rank of sub-lieutenant, and midshipman evolved into naval cadet.
There are two petty officer ranks in the Royal Canadian Navy. Petty officer, 2nd class (PO2) (maître de deuxième classe or m2 in French) is equivalent to a sergeant and petty officer, 1st class (PO1) (maître de première classe or m1) is equivalent to a warrant officer.
Petty officers are normally addressed as "Petty Officer Bloggins" or "PO Bloggins", thereafter as "PO". The "1st class" and "2nd class" designations are normally only used when such a distinction needs to be made, such as on a promotion parade or to distinguish two petty officers with similar names but different ranks. The NATO rank denotion for "petty officer, 2nd class" is OR-6 (petty officers, 2nd class with less than 3 years seniority are considered OR-5). The NATO rank denotion for "petty officer, 1st class" is OR-7.
A petty officer is a non-commissioned officer in the Indian Navy, equivalent to the NATO rank enlisted grade of OR-5. They are equal in rank to a sub inspector of police in the Indian Police Services, or sergeant in the Indian Army and Indian Air Force. A petty officer is superior in rank to a leading rate and subordinate to a chief petty officer, as is the case in the majority of Commonwealth navies.
A petty officer has the ability to work as a leader, capable of taking charge of a group of personnel, and taking roles in the training and recruitment of new members of the Indian Navy.
In the Royal Navy, the rate of petty officer comes above that of leading rating and below that of chief petty officer. It is the equivalent of sergeant in the Royal Marines, British Army and Royal Air Force. Petty officer is the lowest of the senior rating grades. Petty officers, like all senior rates, wear "fore and aft" rig.
The title of petty officer in the United States Navy and United States Coast Guard has three separate "classes" (3rd class through 1st class) and three senior grades (chief petty officer, senior chief petty officer, master chief petty officer). Petty officer, first class is equivalent in paygrade to staff sergeant in the United States Army and Marine Corps, and technical sergeant in the United States Air Force. Petty officer, second class is equivalent in paygrade to sergeant in the United States Army and Marine Corps, and staff sergeant in the United States Air Force. Petty officer, third class is equivalent in paygrade to corporal in the United States Army, corporal in the United States Marine Corps, and senior airman in the United States Air Force.
Enlisted rank has two components: rate (pay grade) and rating (job specialty). Both components are reflected in the title. A sailor in the rate of petty officer first class with a rating of yeoman, would be a Yeoman 1st Class (abbreviated "YN1"). In the Navy, it is acceptable to refer to Petty Officer as such, while in the Coast Guard, rating is always used.
In some countries the same term is used as for a non-commissioned officer (NCO) in land forces, e.g. "suboficial" in some Spanish-speaking countries. The Russian equivalent is Glavny Starshina.