Sanity was the opposite of insanity, which described the state of having a healthy mind and not being mentally ill.
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?")