A lock of hair is a piece or pieces of hair that has been cut from, or remains singly on, a human head, most commonly bunched or tied together in some way. A standard dictionary definition defines a lock as a tress, curl, or ringlet of hair (dictionary.com).
A primitive belief maintains that owning a lock of hair from another's head gives one power over that individual, in the same manner that owning a piece of clothing or image of an individual grants the owner such powers.
Historically, giving a lock of ones hair to someone has been considered a sign of love and devotion, especially before an impending separation. It is still a popular trope in fiction, particularly the romance genre.
During antiquity, Roman girls who were about to be married offered locks of hair to Jove (Jupiter) in his forest god aspect, Virbius (Virbio).
An ancient and worldwide (e.g. China, Egypt, Thailand, Albania, Ukraine, India, Israel, etc.) pre-adolescent custom was to shave children's heads but leave a lock of hair (sometimes several locks) remaining on their heads. Upon reaching adulthood the lock of hair was usually cut off (see Rites of passage).
Riffian (Berber) men of Morocco had the custom of shaving the head but leaving a single lock of hair on either the crown, left, or right side of the head, so that the angel Azrael is able "...to pull them up to heaven of the Last Day."
A common superstition holds that a lock of hair from a baby's first haircut should be kept for good luck.
An old Irish superstition holds that it is unlucky to accept a lock of hair (or a four-footed beast) from a lover.
A lock of Beethoven's hair, cut from his head in 1827, was auctioned in 1994 through Sotheby's of London. Research on the hair determined that the composer's lifelong illness was caused by lead poisoning.
A Polish plait (Koltun in Polish, meaning "Knot", but often referred to in English as an "Elf-Lock") is a lock of matted hair similar to a dreadlock. Due to a scalp disease (Plica polonica), King Christian IV of Denmark (1577–1648) had a Polish plait hanging from the left side of his head, adorned with a red ribbon. His courtiers were said to have adopted the hairstyle in order to flatter the king. Due to superstitious beliefs, the Polish plait used to be particularly common in Poland (hence its name). Initially, the plait was treated as an amulet, supposed to bring good health, as the plait was supposed to take the illness "out" of the body, and therefore it was rarely cut off.
From the end of the 16th century until well into the 17th century it was popular amongst European "men of fashion" to wear a lovelock. The lovelock was a long lock of usually plaited (braided) hair made to rest over the left shoulder (the heart side) to show devotion to a loved one.