A single girl (or, rarely, a boy) who helps a host family with childcare and/or housework while staying as a guest with a host family and generally receiving a small allowance (or pocket money)
An au pair is a young person from a foreign country who lives with a family in order to learn the language and who helps to look after the children. a young person, usually a woman, who stays with a family in a foreign country to learn the language, and looks after their children for a small wage