Familiar and somewhat affectionate term for a young girl or young lady, often used by older people or in a friendly, informal context. The ちゃん suffix softens the word 嬢 (じょう), which on its own can mean 'young woman' or 'miss'.
The young girl in the neighborhood always greets me cheerfully.
The shop clerk called me 'young lady'.
The old man gave a candy to the young lady who came to the shop.
少女 is a more literary or formal term for 'young girl', while 嬢ちゃん is a casual, affectionate way of referring to or addressing one.
お嬢さん is the polite, respectful way to refer to someone's daughter or a young lady, while 嬢ちゃん is a more casual, affectionate version often used with children.
Derived from 嬢 (じょう, 'young woman') with the diminutive suffix ちゃん, which expresses familiarity or affection. The exact historical origin of 嬢 is uncertain, but it has been used in Japanese to refer to unmarried women or daughters.