Corbomite was an imaginary substance invented by Captain 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, Balok's massive Fesarius. Threatened with imminent destruction, Kirk seized upon the chance comments of Dr. McCoy and Spock for inspiration. On-the-fly, he "explained" the nature of corbomite to his opponent: that it reacts to any destructive energy that touches a 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, ultimately revealing Balok's power and belligerence were similar exaggerations. (TOS: "The Corbomite Maneuver")
A year later, Kirk and his senior officers were stricken with a hyper-aging radiation sickness, forcing the inexperienced Commodore George Stocker to take command. Stocker brought the Enterprise into dire peril by ordering the starship into the Romulan Neutral Zone, inviting Romulan attack. By the time Kirk was cured, the starship was under fire from up to ten Birds-of-Prey. Kirk resorted to his corbomite bluff by sending a message to Starfleet in a broken code, warning that activation of the "corbomite device" would destroy the Enterprise and all matter within 200,000 kilometers, leaving behind a "dead zone" to be avoided for the next four years. The Romulans fled the area, allowing the Enterprise to escape. (TOS: "The Deadly Years")