Strictly speaking, a Subaltern is any officer below Field Officer rank - which starts at Major (unless you are the Adjutant).
However, since we Brits are happy to encourage pompous Junior Officers to think excessively highly of themselves despite their general ignorance of strict military terminology, the word Subaltern is commonly used to refer only to those at the bottom of the commissioned-officer food-chain, meaning the 2nd Lieutenant(or one-pip-wonder) and the Lieutenant.
They range in quality from utterly clueless to (particularly for non-grads) very experienced, capable soldiers. Grads don't really have time for that bit and carry "utterly clueless" with them until well into their Captaincy.
Well, at least that's what Team Non-Grad would have you believe, to make up for the enormous chips they all carry around with them.
By the way, Lieutenant is, despite being a French word (meaning something along the lines of 'the bloke who watches the fort while the real boss is away'), pronounced LEFF-tenant. Anyone pronouncing it loo-TENANT is either a) French b) a Septic or c) a mong.
Theft of the term
The term was pinched by a bunch of postcolonialist poseurs to refer to junior outsider components of the colonial power structure, as in "Can the Subaltern Speak ?", an incomprehensible pile of shite written by Gayatri Chakravorty Spivak.
|Second Lieutenant | Lieutenant | Captain | Major | Lieutenant Colonel | Colonel | Brigadier | Major General | Lieutenant General | General | Field Marshal|
|Private | Lance Corporal | Corporal | Sergeant | Staff Sergeant | Warrant Officer Class 2 | Warrant Officer Class 1|