A reprimand, in Starfleet, is a serious formal censure that is included in a person's service record. A commanding officer can reprimand a member of Starfleet for violations of the law, regulations, or other unwarranted behavior on the part of the crew member. A reprimand can have a serious negative effect on a person's career, such as slowing or even stopping their upward movement through the chain of command, making them ineligible for certain positions, or demotion to a lower rank.
When Demora Sulu was apparently killed by Captain John Harriman, her godfather Pavel Chekov was quite angry with Harriman for her death. After her funeral service Chekov proceeded to get into a fight with Harriman. In the aftermath of their quarrel, a disciplinary board entered a formal reprimand in Chekov's service record, and ordered him to make a written apology to Captain Harriman. It was later revealed that it was not Demora who died, but a mindless clone created by an old enemy of her father Hikaru Sulu. The reprimand meant that it would be a number of years before Chekov would gain his own command. (TOS novel The Captain's Daughter).
Captain Jean-Luc Picard would be forced to enter a reprimand in Lieutenant Worf's record after Worf killed Duras, son of Ja'rod. This was in response to Duras murdering K'Ehleyr, and the Klingon Empire felt that Worf had acted within the bounds of Klingon law and tradition. However Captain Picard felt that he could not condone such killing by anyone wearing the uniform, and entered a reprimand in Worf's record. (TNG episode Reunion and StarTrek.com Biography of Worf)
Starfleet Intelligence later entered another reprimand in Worf's record when he abandoned a mission to retrieve his critically injured wife Jadzia Dax. This resuled in the death of a Cardassian defector with valuable intelligence on Founder activities in the Alpha Quadrant. Because of the covert nature of the mission, no formal charges were filed, but Captain Sisko believed that Worf would have a hard time ever gaining his own command. (DS9 episode Change of Heart and StarTrek.com Biography of Worf). While Worf's progression through the ranks was halted for a number of years, his stellar performance in the following years led to Admiral Leonard James Akaar placing Worf on the short list for promotion to Captain in 2387.
Following the revelations of Captain Jean-Luc Picard's involvement in forcing President of the United Federation of Planets Min Zife from office, an Article thirty-two hearing was convened on Earth to determine if Picard should be subject to a formal court martial. Ultimately Admiral Marin Liu determined that criminal charges were not warranted and did not recommend court martial. She did however feel that Picard still needed to be punished, and after consulting with Admiral Leonard James Akaar and other Federation officials placed a reprimand in Picard's file. Liu stated that while Picard would be allowed to continue serving in Starfleet, he would be permanently barred from being promoted to Flag officer. (TNG novel: Collateral Damage)