Ugg boots (sometimes called uggs) are unisex sheepskin boots, lined with wool on the inside and with a tanned outer surface. The UGG name and variants thereof have been brands since 1971, and in some countries the Ugg trademark is still upheld, but in Australia and New Zealand the trademark was removed from the registry and ugg is considered a generic name of a style of boot.
Ugg boots often have a synthetic sole, although this is not universal. Heights range from around the ankle to above the knee, they are available in a range of different colours as both slip-on and lace-up varieties, and they are produced by a number of manufacturers. The natural insulative properties of sheepskin gives thermostatic properties to the boots: the thick fleecy fibers on the inner part of the boots allow air to circulate and keep the feet at body temperature. This means that ugg boots can be worn without socks even in relatively cold weather.
The ugg boot style is believed to have originated in Australia or New Zealand, although the exact date is uncertain—they have been, at different times, identified with pilots in World War I who are said to have worn them for warmth in unpressurized planes, shearers in rural Australia during the 1920s, surfers and competitive swimmers who wore them in the 1960s and 1970s for keeping warm while out of the water. In the early 2000s, they became a fashion trend in the United States, leading to increased global sales.