Demotion

A Demotion is a punitive reduction in a person's rank. A member of the United Federation of Planets Starfleet can be reduced in rank for serious violation of laws or regulations.

Following the disastrous first contact between the Federation and the Talin people, both Spock and Uhura were reduced to the rank of Ensign. Spock chose to resign from the fleet instead of remaining in at that reduced rank. When it was discovered that an alien force had been interfering in Talin society, Spock and the remainder of the Enterprise crew were restored to their former ranks and positions. (TOS novel Prime Directive).

In the 23rd century, James T. Kirk was reduced from the rank of Admiral to Captain as punishment for his actions in disobeing the Starfleet Commander in rescuing Captain Spock from the Genesis Planet. However the council also ordered that Kirk be returned to starship command. (Movie Star Trek IV: The Voyage Home).

Lieutenant Tom Paris was demoted to the rank of Ensign in 2375 for disobeying the orders of Captain Kathryn Janeway and sentenced to thirty days in the brig. (VOY episode Thirty Days). At the end of 2376 Paris was reinstated as a Lieutenant. (VOY episode Unimatrix Zero, Part I).