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second-in-command (EN)
noun

noun “second-in-command”

singular second-in-command, plural seconds-in-command
  1. (in military) an officer who is next in rank below the commanding officer
    The major served as the second-in-command to the colonel in the battalion.
  2. a person who is next in authority after the leader of a group or organization
    He was appointed as the company's second-in-command to assist the president with daily operations.