Longman Dictionary of Contemporary English
Wiktionary
n. 1 Somebody ranking next below a commander. 2 Someone ranking next below the person in charge.
WordNet
n. someone who relieves a commander
Wikipedia
The second-in-command (2i/c or 2IC) is the deputy commander of any British Army or Royal Marines unit, from battalion or regiment downwards. He or she is thus the equivalent of an executive officer in the United States Army. This terminology is also used in many other Commonwealth armies.
The second-in-command of a battalion is usually a major and the second-in-command of a regiment is a lieutenant colonel. The second-in-command of a company, squadron or artillery battery (in which he or she is called the battery captain) is usually a captain (although infantry company second-in-commands were usually lieutenants until after the Second World War), the second-in-command of a platoon or troop is the platoon or troop sergeant, and the second-in-command of a section is usually a lance corporal.
In the Royal Navy, the second-in-command of a vessel is known as the first lieutenant or executive officer.
Usage examples of "second-in-command".
The view that Adams was unsuited to prepare the nation for war and that Hamilton, by contrast, was the ideal choice for second-in-command was shared by McHenry and Secretary Pickering alike.
Second-in-Command, but by one of his underlings his ascosporic twin differing from him only in strength of will and force of personality.
Lieutenant Priddie, second-in-command of the lower gundeck, lay face down, his back pierced with long splinters which had been blasted from the planking.
I was accompanied by Issa, my second-in-command, and the two of us stood draped in our long dark cloaks beside the gate.
And if this impression seems partisan, it is confirmed by his years as Subregulus, as second-in-command to Edward.
When her mission on Tahiti had been accomplished and she was westward bound, among the islands of the Tongan Group, Fletcher Christian, second-in-command of the vessel, raised the men in revolt against Captain William Bligh, whose conduct he considered cruel and insupportable.
NSA, Strathmore had skyrocketed from head of Crypto Development to second-in-command of the entire NSA.
Then Axis had simply been the BattleAxe of the Seneschal and Belial his second-in-command.
In return for Blair support in this appointment, the pompous old hero had agreed to take on Congressman Frank Blair as second-in-command.
Rao Sastri, as Celia knew, was the nucleic acid chemist-a Pakistani, formerly a Cambridge colleague-whom Martin had recruited as his scientific second-in-command.
Aramina had been fascinated, and obliquely repelled by Giron, Thella's second-in-command, the dragonless man who had scrutinized her so intently that Aramina had been hard put not to squirm under his cold and empty eyes.
I'm designating Captain Nash as my second-in-command with the special grade of Commodore for the duration of this incident.
With Tyne Aldritch dead, all was in disarray, and Tyne’s second-in-command, the stolid, unimaginative Droy of Eastlake would not have been able to salvage things even if he had lived.
And who would be a more logical choice as the triad's new second-in-command than Seemal Tezlin, who had served his acquisitor so admirably at Beta Cabrini?
Awkwardly, the second-in-command cleared his throat, then offered: "We're approaching Amargosa, Captain.