LATCH-KEY CHILD noun: a child who is at home without adult supervision for some part of the day, especially after school until a parent returns from work. This term was usually given to children that carried their home keys around their neck or in their pockets.
This expression was developed in 1942; when fathers were off to World War II and mothers had to find a job for the first time in their lives. The phrase became most popular for the Generation Xers. Divorce was on a rise and single parents had no other way then to leave their children at home.