This word is perhaps best translated as 'coziness', but the English word can only express one aspect of the German meaning. It is a great word for winter and for the Holidays in particular.
Gemütlichkeit is epitomized by a snug room with a sofa nestled next to a roaring open fire, and the friendly, jovial atmosphere, and the resulting state of mind.
Germans use it to refer to many things from people, to a beer in a warm pub, to evenings watching a film.
The OED defines Gemütlichkeit as "the quality of being pleasant cheerful; cozy, snug, homely; genial, and good-natured".
Even though it is an accepted word in English, it is yet to find its way into everyday language. Regardless, my wish to you all for this Holiday Season is Gemütlichkeit!