Wiktionary
n. second in command of a military unit or ship
WordNet
n. the officer second in command
Wikipedia
An executive officer (often abbreviated XO) is generally a person responsible for running an organization, although the exact nature of the role varies depending on the organization.
Usage examples of "executive officer".
Newman had already displaced ISETs Alpha and Bravo forward to Siirt as the QRE He placed Bart Coombs in command of the QRF, with Captain Bruno Macklin, the SAS officer, as Executive Officer.
It had been that way in the Navy, too, when she was my executive officer.
Now, sensing how curt he'd just been to his executive officer, he turned with a smile on his face.
Although it was intended only for the eyes of the captain and the executive officer, I will be happy to explain as much as I can to any member of this crew who feels the need to know more about it.
It was one of the qualities which made him an outstanding executive officer.
Gaunt was an efficient executive officer, the sort who always got the details right and developed an almost uncanny ability to anticipate his CO's desires.