A safe or vault was a secure compartment used to protect the objects within it against theft or damage. Money or other valuable objects were typically placed within safes for this purpose. Safes were secured with a locking mechanism, which could be opened with a key, combination lock, or other technology.
A group of Ferengi led by Ulis demanded to know the location of the vault on board the Enterprise NX-01 from Jonathan Archer after seizing the ship. They did not believe him when he denied such a thing existed, but used it as part of a ruse to regain control of the ship. (ENT: "Acquisition")
Captain Kirk had a safe in his quarters on the Enterprise, in which he kept his command packet and a record tape with his final orders to Mister Spock and Doctor McCoy should he ever be declared dead. (TOS: "What Are Little Girls Made Of?", "The Tholian Web")
The casino within an altered version of Vic's lounge had a safe. As part of a scheme to return the holoprogram to normal after tripping a jack-in-the-box, Odo and Nog were required to break into the safe to steal the money within. This safe was secured with a combination lock, which Nog planned to break using his sense of hearing. However, it had an auto-relock tumbler designed to prevent just what he was attempting. Despite that, Nog was able to successfuly open the safe and get away with the money. (DS9: "Badda-Bing, Badda-Bang")