A thought-shield was a type of force field capable of blocking mental abilities. It also had the unintended side effects of inducing strong emotions and hallucinations. Prolonged exposure would eventually result in death.
In 2269, the Klingons developed thought-shield technology and erected one around Organia, preventing the Organians from leaving it or learning what was happening outside of it. This allowed the Klingon Empire to declare war against the Federation without Organian interference. The USS Enterprise attempted to transport a copy of Spock made of tachyons to Organia, but the thought-shield reflected the tachyons back to the ship, creating a mirror duplicate. After reaching Organia months later, James T. Kirk, Spock and Montgomery Scott beamed down to the planet, where, despite the shield's effects, they disabled the thought-shield and freed the Organians. (TOS novel: Spock Must Die!)