A vice-admiral is a very senior officer in a navy — in Britain and the United States, one of the top ranks. The word names both the rank itself and the person holding it, so you can say 'he was promoted to vice-admiral' and 'Vice-Admiral Nelson gave the order'. In the navy the admiral ranks go, from lower to higher: rear admiral, then vice-admiral, then admiral, then (in wartime or ceremony) admiral of the fleet. So a vice-admiral is above a rear admiral and below a full admiral. A person at this level commands a fleet or a large part of the navy, not a single ship. Note the odd thing about 'vice' here: it does not mean the person is second-in-command to one particular admiral, the way a vice-president serves under one president. It is simply the name of a rank, and there are many vice-admirals at once. As a title before a name it is capitalised and often shortened in writing to 'Vice Adm.' or 'VAdm'. British spelling usually keeps the hyphen; American writing often drops it and writes 'vice admiral'. You will meet this word mostly in history, news about the navy, and films — it is not an everyday word.