A jersey is an item of knitted clothing, traditionally in wool or cotton, with sleeves, worn as a pullover; this is to say, it does not open at the front, unlike a cardigan. It is usually close-fitting and machine knitted in contrast to a guernsey that is more often hand knit with a thicker yarn. The word is usually used interchangeably with sweater and is the name always officially used in the United Kingdom to refer to the item when worn as part of the uniform of military, police or other uniformed personnel.
The English Channel Island Jersey was famous for its knitting trade in medieval times, and because of that original fame, the name jersey is still applied to many forms of knitted fabric, round or flat.
The traditional jersey, and traditional guernsey, are dyed a navy blue colour, using a dye that does not require stripping the wool of its natural oil, rendering these sweaters surprisingly water resistant. The wool used in guernseys is often passed through oil in order to render it doubly water-resistant.
Alternatively, a jersey as used in sport is the name of a shirt worn by a member of a team, usually oversized, typically depicting the athlete's name and team number as well as the team's logotype or corporate sponsor. Cycling jerseys of specific colour or pattern represent certain statuses in cycling - such as the maillot jaune (yellow jersey) of the leader of the Tour de France, or the rainbow jersey. Cycling jerseys are usually made of synthetic microfiber material to aid in wicking sweat away from the skin to allow it to evaporate. The main garment of an ice hockey uniform, which was traditionally called a sweater, is increasingly know as a hockey jersey, and has become a fashion accessory. Basketball jerseys are usually sleeveless. Another kind of jersey is the pennie (soccer).
It is common for Australian Year 12 students and Year 13 students in New Zealand to receive personalised jerseys during their last year at school - these are commonly referred to as a Year or Leavers' Jerseys. They are based on the design of a sport jersey (usually Rugby Union, feature school colours, the year the students graduate, and a personalised nickname. They are usually worn over the normal school uniform.
In some sports, such as basketball and American football, a player's jersey may be "retired". When a jersey is retired that player's jersey is placed usually where the team plays in honor of that player's accomplishment. The retiring of a jersey at one time included the retiring of the number on the player's jersey leaving it unavailable for future players, to further honor that player. However as jerseys are usually double digit (00-99) that leaves only 100 possible numbers. As such the practice of retiring the number has dwindled, and in most cases only the specific jersey is retired and put on display.