Teller light is an
atomic detonation effect. It is the name given to the
fluorescence that can be seen surrounding an
atomic device at the very opening
picoseconds of its
detonation. The Teller light is caused by the core reaching
critical mass, at which point a flood of
prompt gamma radiation escapes it and causes the surrounding air molecules to fluoresce. This light will appear before the actual weapon disintegrates. At later stages of the detonation, fluorescence occurs due to the interaction between
fast neutrons and the air; this is also referred to as Teller light.
The name is from Edward Teller, a Hungarian physicist known as the 'father of the hydrogen bomb.' Multistage fusion weapons are known (in the U.S.) as Teller-Ulam designs after Teller and his Polish colleague Stanislaw Ulam, a mathematician.
Note that the Teller light is initially caused by a fission core going critical - not by a fusion reaction, despite Teller's association with the latter. Fusion weapons contain fission triggers (referred to as the primary) and hence will emit Teller light when detonated, however.