The security override code was a specific command code used to override security protocols on a starship or starbase.
In 2365, Captain Jean-Luc Picard made a security override request to gain remote control access over the USS Lantree. His access code to complete the task was "omicron-omicron-alpha-yellow-daystar-2-7." This type of access overrode control of the Lantree's bridge, allowing them to shutdown its engines, and access the ship's main viewer. (TNG: "Unnatural Selection")
Later that year, during Operation Lovely Angel, the USS Hathaway's first officer Lieutenant Worf's knowledge of the USS Enterprise-D's security override, allowed him to convince the Enterprise's sensors that an enemy ship was approaching. This caused the ship's instruments, specifically the sensor codes, to "lie to them," and therefore helped give the Hathaway an edge during the battle simulation. Unfortunately, after resetting Enterprise's, a Ferengi warship observed the two Federation starships battle exercise, and the Enterprise, believing the incoming warship was another ruse, ignored the imminent threat, and was thereafter disabled. (TNG: "Peak Performance")
In 2366, Worf used the priority one security override to enter Captain Picard's quarters. (TNG: "Allegiance") He later called Security, in 2367, to override the lock to Deanna Troi's quarters after he heard her scream when he was outside in the adjacent corridor. (TNG: "Clues")
When the Kaelon attempted to abduct Jadzia Dax from Deep Space 9 in 2369, those involved were able to disable the station's force fields as necessary, indicating that all of the stations security override codes. (DS9: "Dax")
In 2370, Deanna Troi used a security override to enter Calloway's quarters, gaining access with the authorization code "Troi-delta-2-9." (TNG: "Eye of the Beholder")
In 2373, Chief Miles O'Brien initiate security override to scramble DS9's surveillance channels 4A, 4B, 4C and 4D, used to oversee the station's holding cells, with the authorization code "O'Brien-7-1-2." When he was later discovered by Odo, O'Brien explained he did it because "The fiber optic relay was out of sync," and "The isolinear chip junction was..." before being cut-off by Odo, who knew he was lying and told him his attempt to do whatever it was he was doing was sloppy. (DS9: "The Assignment")
In 2375, Lieutenant B'Elanna Torres used a security override to attempt to discover the author of the holographic program Insurrection Alpha. (VOY: "Worst Case Scenario")
In 2377, Tuvok used the security override "Tuvok-pi-alpha" to gain access to Ensign Tabor's quarters. (VOY: "Repression") Later, he used the same security override, "Tuvok-pi-alpha", to open the transporter room doors, during an attempted hijacking by the Klingons, and nearly once again, after Neelix had locked Tuvok out of their shared quarters. (VOY: "Prophecy") Later that year, Tuvok overrode the holodeck doors using the security override "Tuvok-sigma-4". (VOY: "Renaissance Man")