Corbomite is an imaginary substance created by James T. Kirk as a bluff against attack. Describing it as "a material, and a device, which prevents attack on us", Kirk used the corbomite bluff on at least two occasions.
In 2266, the USS Enterprise was captured by a vessel of the First Federation, the Fesarius. Threatened with imminent destruction, Kirk seized upon a chance comment by Doctor McCoy. He concocted the story of corbomite, and "explained" to Balok that this substance reacts to any destructive energy that touches an Federation vessel, creating a reverse reaction that destroys the attacker. He went on to advise Balok that since the introduction of corbomite more than two centuries ago, no attacking vessel had survived the attempt. The bluff was successful, although it later emerged that the threat was merely a test. (TOS: "The Corbomite Maneuver")
A year later, Kirk used the corbomite bluff against a group of Romulan battlecruisers. Having strayed into the Romulan Neutral Zone after becoming infected with an aging virus on Gamma Hydra IV, Kirk warned the Romulans away by claiming to have a "corbomite device" which would destroy all matter within 200,000 kilometres. (TOS: "The Deadly Years")