An encryption code was a method used encrypt data held within a computer from being accessed by an outside intruder.
In 2151, Hoshi Sato proposed that it would be easier breaking a Vulcan encryption code than getting the information what Malcolm Reed's favorite food is. (ENT: "Silent Enemy")
Seven of Nine used Borg encryption codes to access the turbolifts and blocked Harry Kim's commands on her way to the shuttlebay on deck 10. (VOY: "The Raven")
In 2374, Seven of Nine used such an encryption code when initializing the transport of the Alpha-Hirogen and the member of Species 8472. Harry Kim was unable to stop this transport due to the Borg encryption code. (VOY: "Prey")