Semantics

Волосы у неё были распущены. She had let her hair down.
Его загорелая грудь была совершенно лишена волос. Может быть, они так выгорели на солнце, что были просто не видны. His tanned chest was completely hairless. Perhaps the hair was so bleached out by the sun that it was just not noticeable.
This entry is primarily concerned with the word волосы denoting the covering of hair on the head and body of humans. A different word, шерсть, is used to refer to the covering of hair on the skin of animals. Most of the expressions in this entry concern hair on the head; special notes are provided when a given expression can refer to body hair, or is restricted to either hair on the head or on the body. If there is no indication of this kind, common sense will dictate whether or not a given expression can refer to body hair.

The word волосы in the meaning "hair, a covering of hair" occurs only in the plural. However, волосы is also the Nominative and Accusative plural forms of волос in the meaning "a hair," e.g. На подушке лежали чьи-то чёрные волосы, и она их брезгливо стряхнула на пол "There were some black hairs on the pillow, and she brushed them to the floor in disgust" or выдёргивать седые волосы "to pull out grey hairs." This entry includes this latter meaning in cases where properties such as size, color, etc. are relevant.