The full-body environmental suit and snouted helmet worn by every Breen seen in public, long assumed to keep its wearer cold and hiding the species' true appearance for centuries.
The refrigeration suit is the sealed armour and visored helmet that the Breen wear at all times among outsiders. For most of Star Trek's history no one outside the Breen Confederacy knew what lay beneath it, and the common belief was that the suits kept their wearers at the freezing temperatures of their home.
The suits first appear in the Deep Space Nine episode Indiscretion, where Kira and Dukat steal a pair to slip into a Breen-run mine on Dozaria. They return throughout the final year of the Dominion War, when Weyoun casts doubt on the cold-world story by remarking that the Breen homeworld is quite temperate.
Discovery's fifth season finally looks inside the helmet through the character L'ak, showing that the Breen have both a solid form and a translucent natural one, and that the suit sustains the latter.