N.E.S.T. refers to a team of technicians, scientists and engineers within the US government prepared to deal with any type of nuclear incident.
When a nuclear bomb brought by Second Wave was found at Norton Airfield, a NEST team arrived to try to disarm it. One of their technicians discovered the device was a decoy until they found the real device. The other technician tried to disarm it but he was unsuccessful. The bomb was then detonated safely at the Mojave Desert.
When a threat concerning nuclear rods was discovered on New York City, CTU ordered to have appropriate intelligence forwarded to the NEST team stationed at Fort Hamilton. He also told Agent Cole Ortiz that as soon as the nuclear rods were recovered they were to be transported to the fort. When the thread materialized and a bomb was left by the terrorists in an alley in Manhattan, a N.E.S.T. technician arrived at the site and used a robot to secure it.