The meaning of lieutenant differs in different military (see comparative military ranks), but is often subdivided into senior (first lieutenant) and junior (second lieutenant) ranks. In navies it is often equivalent to the army rank of captain; it may also indicate a particular post rather than a rank. The rank is also used in fire services, emergency medical services, security services and police forces.
Lieutenant may also appear as part of a title used in various other organisations with a codified command structure. It often designates someone who is "second-in-command", and as such, may precede the name of the rank directly above it. For example, a "lieutenant master" is likely to be second-in-command to the "master" in an organisation using both ranks.
Political uses include lieutenant governor in various governments, and Quebec lieutenant in Canadian politics. In the United Kingdom, a lord lieutenant is the sovereign's representative in a county or lieutenancy area, while a deputy lieutenant is one of the lord lieutenant's deputies.
The word lieutenant derives from French; the lieu meaning "place" as in a position (cf. in lieu of); and tenant meaning "holding" as in "holding a position"; thus a "lieutenant" is a placeholder for a superior, during their absence (compare the Latin locum tenens).
In the 19th century, British writers who considered this word either an imposition on the English language, or difficult for common soldiers and sailors, argued for it to be replaced by the calque "steadholder". However, their efforts failed, and the French word is still used, along with its many variations (e.g. lieutenant colonel, lieutenant general, lieutenant commander, flight lieutenant, second lieutenant and many non-English language examples), in both the Old and the New World.
Pronunciation of lieutenant is generally split between the forms // ( listen) lef-TEN-ənt and // ( listen) loo-TEN-ənt, with the former generally associated with the armies of the United Kingdom and Commonwealth countries and the Republic of Ireland, and the latter generally associated with anyone from the United States or other western countries. The early history of the pronunciation is unclear; Middle English spellings suggest that the /luː-/ and /lɛf-/ pronunciations may have existed even then. The rare Old French variant spelling luef for Modern French lieu ('place') supports the suggestion that a final of the Old French word was in certain environments perceived as an . Furthermore, in Latin, the lingua franca of the era, the letter v is used for both u and v.
In Royal Naval tradition—and other English-speaking navies outside the United States—a reduced pronunciation // ( listen) is used. This is not recognised as current by recent editions of the OED (although the RN pronunciation was included in editions of OED up until the 1970s).