Noun
(military, navy) A commissioned officer of the navy whose rank is immediately below that of a lieutenant.
(UK) A commissioned officer of the Royal Navy ranking above a midshipman and below a lieutenant.
(Australia, Canada, New Zealand) A commissioned officer of the navy ranking above an acting sub-lieutenant and below a lieutenant.
Source: en.wiktionary.orgChalmers, p. 12 Beatty left HMS Alexandra in March 1889 and joined the cruiser HMS Warspite in July 1889 for manoeuvres before joining the sailing corvette HMS Ruby in September 1889, in which he was promoted to sub-lieutenant on 14 May 1890. Source: Internet
In 1914, he was promoted to the ranks of lieutenant in the army and sub-lieutenant in the navy, then to captain and lieutenant in 1916. Source: Internet
Ziegler 1986, p46 He was appointed Executive officer (second-in-command) of the small warship HMS P.31 on 13 October 1918 and was promoted sub-lieutenant on 15 January 1919. Source: Internet