The first documented tea cosy in Britain was in 1867 and it was probably the Duchess of Bedford, who by establishing the activity of afternoon tea in 1840, increased the popularity of tea cosy.
They then flourished during the late 19th Century and were also starting to be used in North America during the same period.
It is defined in Wikipedia as ‘a cover for a teapot, traditionally made of cloth or wool, which is used to insulate the tea, keeping it warm while it brews’.
Cloth tea cosies often have padded inserts which can be washed separately and many are hand knitted looking just like a woollen hat with a bobble on the top.
Of course they don’t have to be knitted you can buy tea cosies made from fabric like this tweed fabric one from Folksy at £28