Fogbank (stylized as FOGBANK) is a code name given to a secret material used in the W76, W78 and W88 nuclear warheads that are part of the United States nuclear arsenal. The process to create Fogbank was lost by 2000, when it was needed for the refurbishment of old warheads. Fogbank was then reverse engineered by the National Nuclear Security Administration (NNSA) over five years and at the cost of tens of millions of dollars. 
Fogbank's precise nature is classified; in the words of former Oak Ridge general manager Dennis Ruddy, "The material is classified. Its composition is classified. Its use in the weapon is classified, and the process itself is classified." Department of Energy Nuclear Explosive Safety documents simply describe it as a material "used in nuclear weapons and nuclear explosives" along with lithium hydride (LiH) and lithium deuteride (LiD), beryllium (Be), uranium hydride (UH), and plutonium hydride.
However, NNSA Administrator Tom DAgostino disclosed the role of Fogbank in the weapon: "Theres another material in the—its called interstage material, also known as Fogbank", and arms experts believe that Fogbank is an aerogel material which acts as an interstage material in a nuclear warhead; i.e., a material designed to become a superheated plasma following the detonation of the weapons fission stage, the plasma then triggering the fusion-stage detonation.