The hat Santa Claus is depicted as wearing is a stocking cap. A traditional stocking cap has a conical shape, is long and normally features a pompom or tassel at the end. Stocking caps are normally knitted; Santa's is made from fur, according to "A Visit from St. Nicholas."
Many people use the term "stocking cap" to generalize knit caps as a whole, but most knit caps have specific names. Santa's hat can sometimes be mistaken for a traditional nightcap, which had a similar design, but an entirely different function. Nightcaps were worn with pajamas as sleeping attire. The long top could be wrapped around the neck for warmth, but it was too short to cause choking.