Mental discipline was the ability to consistently engage in mental practices to achieve goals, even when it was difficult or would require sacrifice.
In 2268, although Human, Miranda Jones had never visited the planet Earth; she did, however, spend four years on Vulcan. While on Vulcan, she studied Vulcan mental discipline to turn her telepathic abilities "off" in order to maintain her sanity. (TOS: "Is There in Truth No Beauty?")