A helmet-like machine left by the vanished builders of Sigma Draconis VI that temporarily fills the wearer's mind with their advanced scientific and medical knowledge.
The Great Teacher appears in the episode Spock's Brain. The Eymorg women living beneath the surface of Sigma Draconis VI have forgotten the science that built their city, and rely on a machine that pours the old knowledge into anyone who wears it. The learning fades after a few hours.
Their leader Kara uses it to gain the surgical skill needed to remove Spock's brain, which she installs as the Controller that runs the underground complex.
To reverse the operation McCoy puts on the helmet himself and begins reconnecting the brain with complete confidence. The knowledge drains away before he has finished, and he completes the surgery only because Spock, his speech already restored, talks him through the remaining steps.