A glass or metal container, with a vacuum-like space between two walls to prevent heat transfer, often used for storing liquefied gases. Also called a Dewar flask.
The thermos that keeps your coffee hot is a commercial version of the same thing: a container inside another container, with an evacuated area in between as insulation. It isn't a total vacuum - the walls of the container probably couldn't stand up to that.
In an MRI machine, the solenoid generating the magnetic field is inside a dewar of liquid helium.
I have a soft spot for dewars: I used to set up physics lab classes for a living, and one of my first tasks was to fill dewars with liquid nitrogen (for the demos of freezing things and then shattering them). That was a cool job with lots of toys; I would have stayed longer if it weren't for the mercury spills, rusty cans of benzene, misplaced radioactive sources, and sexual harrassment. I didn't mind the iguana that sometimes ran loose in the hall.
Etymology: Late 1800s, named after Sir James Dewar.
Except for similar pronunciation, it has nothing to do with Scotch.