second-in-command

second-in-command (plural seconds-in-command)

1. Somebody ranking next below a commander.
2. Someone ranking next below the person in charge.
   • 1951 June, Dr. W. H. Chaloner, “Alexander Allan's Own Claim”, in Railway Magazine, page 416:

     Allan was first William Buddicom's and then Trevithick's second-in-command, at Edge Hill and at Crewe successively, for the period of 13½ years from February, 1840, to September, 1853, when he left Crewe to become Superintendent of the Scottish Central Railway.

• deputy
• lieutenant