Polish plait (Latin: Plica polonica, Polish: Kołtun polski or plika, Kołtun in Polish meaning matted), less commonly known in English as plica or trichoma, is a particular formation of hair. This term can refer to either a hairstyle or a medical condition, depending on context. The term is connected to a system of beliefs in European folklore, and healing practices in the traditional medicine of the Polish–Lithuanian Commonwealth that believed matted hair was an amulet, or a catchment or trajectory for illness to leave the body.

Plica polonica (synonym, plica neuropathica; common name, "Polish plait") is an uncommon condition in which the hair shaft becomes entangled irreversibly, forming a mass which is matted and sometimes can be sticky and moist.

In this condition, the protective layer of hair (cuticle) is damaged, and the cortex of the hair is exposed. The cortex is a more moist and sticky part of the hair than the outer layer, and the affected hairs adhere to each other and other unaffected hairs. Several factors may contribute to this condition: chemical exposure, hair with natural kinks, hair extensions, quality of water and shampoo, or absence of hair grooming and poor hair care techniques. It may also be caused by or accompanied with lice infestation (pediculosis) leading to inflammation of the scalp, or the mass can become malodorous.

Larry Wolff in his book Inventing Eastern Europe: The Map of Civilization on the Mind of Enlightenment mentions that in Poland, for about a thousand years, some people wore the hair style of the Scythians. Zygmunt Gloger in his Encyklopedia staropolska mentions that Polish plait was worn as a hair style by some people, regardless of gender, in the Pinsk region and the Masovia region at the beginning of the 19th century. He used the term "kołtun zapuszczony" which denotes artificial formation of Polish plait. According to folklore studies today, the style was formed using liquids or wax. Among liquids, a mixture of wine and sugar was used or washing hair daily with water in which herbs were boiled. The most commonly used herb was vinca (Vinca major), followed by Lycopodium clavatum and moss, which caused hair to mat. A similar effect can be had by rubbing hair with wax, or inserting a piece of a candle at the hair ends. Newer Polish dictionaries mention plica as a disease, but the old ones also mention artificially created plica.

In modern times the hairstyle is also known as mono-dreadlock (or mono-dread for short), alluding to how its structure is comparable to a single, massive strand of a dreadlock hairstyle, as well as beaver tail as the mass of hair may resemble the tail of a beaver. The hairstyle can vary in size, from large beaver tails to small plaits.^{[citation needed]}

The Polish plait was quite common in Europe during past centuries when hair grooming was largely neglected. It affected mostly the peasantry, but was not unusual among higher social classes.

Due to superstitious beliefs, the Polish plait used to be particularly common in the Polish–Lithuanian Commonwealth, hence its English and Latin name. Similarly, in German it is called Weichselzopf, or Vistula braid, zopf meaning a braid, and the Vistula being a river in Poland. Initially, the plait was considered an amulet to keep illness away from the body, as it was believed that when disease resolved it left the body to live in the hair, resulting in lessened suffering. For this reason, people not only allowed it to develop, but even encouraged it. According to M. Marczewska, who researched the subject from the perspective of folklore studies, animistic beliefs and long-held pagan beliefs relating to illness viewed illness as caused by an invading evil spirit, which by convalescence left the body and was less problematic when living in the hair formation, which was then shed naturally or cut and ritualistically disposed of by persons specializing in folk medicine or practitioners of folk magic. As people believed that the formation of plica was a sign of resolving of disease, plica, as a hairstyle, was also formed artificially by washing with mixtures of herbs, sweetened wine, waxing, etc.

In the early 17th century, people began to believe plaits were an external symptom of an internal illness. A growing plait was supposed to take the illness "out" of the body, and therefore it was rarely cut off; in addition, the belief that a cut-off plait could avenge itself and bring an even greater illness discouraged some from attacking it. It was also believed that casting a magic spell on someone could cause that person to develop a Polish plait, hence also the name "elflock" was used in English, also Hexenzopf (witches' plait) in German.