A tea cosy (American English tea cozy) is a cover for a teapot, traditionally made of cloth or wool, which is used to insulate the tea keeping it warm while it brews. Tea cosies may be embroidered, perhaps to compliment a fine set of china. Some have been made with hidden pockets to be filled with fragrant herbs or flowers, similar to pot pourri. Tea cosies in fiction include the eponymous items in Edward Gorey's The Haunted Tea Cosy: A Dispirited and Distasteful Diversion for Christmas.
Some other cosies are just evil.

Many tea cosies in Britain are hand-knitted and subsequently resemble woolen hats, even featuring a bobble on top. According to comedian Billy Connolly, if a man is left alone in a room with a tea cosy, and he does not attempt to wear it, he should not be trusted.
The Terrorist Tea Pot is a set including the tea pot and the balaclava tea cosy.

For those of you who like the technical info the pot holds 1.5 litres (50 Oz) which will make you 5 or 6 big cups of steaming hot tea!