The coolant circulating around a starship's warp core, a corrosive gas that liquefies organic tissue on contact and which Picard and Data used to destroy the Borg Queen.
Plasma coolant becomes a weapon in Star Trek: First Contact. Early in the film Picard considers rupturing a coolant tank in the Borg-held engine room of the Enterprise-E, since the substance would dissolve the organic parts of every drone there, but an approach through the Borg defences proves impossible.
In the final confrontation Data, who has seemed to side with the Borg Queen, deliberately misses Cochrane's ship with the torpedoes and smashes the coolant tank with his fist. Green vapour floods the room.
Picard climbs clear on overhead cables while the Queen is dragged down into the gas. Her flesh is eaten away, as is the human skin the Borg had grafted onto Data, leaving her metal skull and spine for Picard to break.