Sentence:  A large thermos-like container called a <WSD>dewar</WSD> contained 645 gallons of liquid helium to be cooled to within two degrees of absolute zero. 
Meaning of the word marked with <WSD> tags:  A vacuum flask; a vessel which keeps its contents hotter or cooler than their environment without the need to modify the pressure, by interposing an evacuated region to provide thermal insulation between the contents and the environment.